Fix PIR term shrinking

[zliu41]

1. Shrinking is sometimes slow because a large number of shrinks are generated. See this comment.
2. There was a test failure on prop_shrinkTermSound :

unpacking sources
unpacking source archive /nix/store/lkm426ykvh3q7y0iqx9lp1zj6pgvpcsb-plutus-root-plutus-core-test-plutus-ir-test-root
source root is plutus-root-plutus-core-test-plutus-ir-test-root
patching sources
building
plutus-ir
  prettyprinting
    basic:                                              OK
    maybe:                                              OK
  prettyprintingReadable
    basic:                                              OK
    maybe:                                              OK
    letInLet:                                           OK
    letDep:                                             OK
    listMatch:                                          OK
    idleAll:                                            OK
    some:                                               OK
    even3:                                              OK
    stupidZero:                                         OK
    mutuallyRecursiveValues:                            OK
    errorBinding:                                       OK
    some:                                               OK
    stupidZero:                                         OK
    recursiveTypeBind:                                  OK
  parsing
    parser round-trip:                                  OK (0.48s)
        ✓ propRoundTrip passed 100 tests.
    parser ignores whitespace:                          OK (2.21s)
        ✓ propIgnores whitespace passed 100 tests.
    parser ignores comments:                            OK (27.73s)
        ✓ propIgnores comments passed 100 tests.
  lets
    letInLet:                                           OK
    letDep:                                             OK
  datatypes
    maybe:                                              OK
    listMatch:                                          OK
    idleAll:                                            OK
    some:                                               OK
    listMatchEval:                                      OK
    dataEscape:                                         OK
  recursion
    factorial:                                          OK
    even3:                                              OK (0.02s)
    even3Eval:                                          OK (0.03s)
    stupidZero:                                         OK
    mutuallyRecursiveValues:                            OK (0.02s)
    errorBinding:                                       OK
  serialization
    serializeBasic:                                     OK
    serializeMaybePirTerm:                              OK
    serializeEvenOdd:                                   OK
    serializeListMatch:                                 OK
  errors
    mutuallyRecursiveTypes:                             OK
    recursiveTypeBind:                                  OK
  names
    renamer does not destroy scoping:                   OK (0.16s)
        ✓ test_scopingGood passed 100 tests.
    scoping bad
      wrong renaming destroys scoping:                  OK
      no renaming does not result in global uniqueness: OK
      renaming with no marking destroys scoping:        OK
  transform
    thunkRecursions
      listFold:                                         OK
      monoMap:                                          OK
      errorBinding:                                     OK
    nonStrict
      nonStrict1:                                       OK
    letFloat
      letInLet:                                         OK
      listMatch:                                        OK
      maybe:                                            OK
      ifError:                                          OK
      mutuallyRecursiveTypes:                           OK
      mutuallyRecursiveValues:                          OK
      nonrec1:                                          OK
      nonrec2:                                          OK
      nonrec3:                                          OK
      nonrec4:                                          OK
      nonrec6:                                          OK
      nonrec7:                                          OK
      nonrec8:                                          OK
      nonrec9:                                          OK
      rec1:                                             OK
      rec2:                                             OK
      rec3:                                             OK
      rec4:                                             OK
      nonrecToRec:                                      OK
      nonrecToNonrec:                                   OK
      oldLength:                                        OK
      strictValue:                                      OK
      strictNonValue:                                   OK
      strictNonValue2:                                  OK
      strictNonValue3:                                  OK
      strictValueNonValue:                              OK
      strictValueValue:                                 OK
      even3Eval:                                        OK
      strictNonValueDeep:                               OK
      oldFloatBug:                                      OK
      outRhs:                                           OK
      outLam:                                           OK
      inLam:                                            OK
      rhsSqueezeVsNest:                                 OK
    recSplit
      truenonrec:                                       OK
      mutuallyRecursiveTypes:                           OK
      mutuallyRecursiveValues:                          OK
      selfrecursive:                                    OK
      small:                                            OK
      big:                                              OK
    inline
      var:                                              OK
      builtin:                                          OK
      constant:                                         OK
      transitive:                                       OK
      tyvar:                                            OK
      single:                                           OK
      immediateVar:                                     OK
      immediateApp:                                     OK
    beta
      lamapp:                                           OK
      lamapp2:                                          OK
      absapp:                                           OK
      multiapp:                                         OK
      multilet:                                         OK
    unwrapCancel
      unwrapWrap:                                       OK
      wrapUnwrap:                                       OK
    deadCode
      typeLet:                                          OK
      termLet:                                          OK
      strictLet:                                        OK
      nonstrictLet:                                     OK
      datatypeLiveType:                                 OK
      datatypeLiveConstr:                               OK
      datatypeLiveDestr:                                OK
      datatypeDead:                                     OK
      singleBinding:                                    OK
      builtinBinding:                                   OK
      etaBuiltinBinding:                                OK
      nestedBindings:                                   OK
      nestedBindingsIndirect:                           OK
      recBindingSimple:                                 OK
      recBindingComplex:                                OK
      pruneDatatype:                                    OK
    retainedSize
      typeLet:                                          OK
      termLet:                                          OK
      strictLet:                                        OK
      nonstrictLet:                                     OK
      datatypeLiveType:                                 OK
      datatypeLiveConstr:                               OK
      datatypeLiveDestr:                                OK
      datatypeDead:                                     OK
      singleBinding:                                    OK
      builtinBinding:                                   OK
      etaBuiltinBinding:                                OK
      etaBuiltinBindingUsed:                            OK
      nestedBindings:                                   OK
      nestedBindingsIndirect:                           OK
      recBindingSimple:                                 OK
      recBindingComplex:                                OK
    rename
      allShadowedDataNonRec:                            OK
      allShadowedDataRec:                               OK
      paramShadowedDataNonRec:                          OK
      paramShadowedDataRec:                             OK
  types
    letInLet:                                           OK
    listMatch:                                          OK
    maybe:                                              OK
    ifError:                                            OK
    mutuallyRecursiveTypes:                             OK
    mutuallyRecursiveValues:                            OK
    nonrec1:                                            OK
    nonrec2:                                            OK
    nonrec3:                                            OK
    nonrec4:                                            OK
    nonrec6:                                            OK
    nonrec7:                                            OK
    nonrec8:                                            OK
    rec1:                                               OK
    rec2:                                               OK
    rec3:                                               OK
    rec4:                                               OK
    nonrecToRec:                                        OK
    nonrecToNonrec:                                     OK
    oldLength:                                          OK
    strictValue:                                        OK
    strictNonValue:                                     OK
    strictNonValue2:                                    OK
    strictNonValue3:                                    OK
    strictValueNonValue:                                OK
    strictValueValue:                                   OK
    strictNonValueDeep:                                 OK
    even3Eval:                                          OK
    sameNameDifferentEnv:                               OK
    typeLet:                                            OK
    typeLetRec:                                         OK
  type-errors
    wrongDataConstrReturnType:                          OK
    nonSelfRecursive:                                   OK
    typeLet:                                            OK
  generators
    prop_genData:                                       OK (8.23s)
      +++ OK, passed 3000 tests.
    prop_genKindCorrect:                                OK (4.93s)
      +++ OK, passed 100000 tests.
    prop_shrinkTypeSound:                               OK (12.88s)
      +++ OK, passed 30000 tests.
    prop_substType:                                     OK (2.55s)
      +++ OK, passed 10000 tests.
    prop_unify:                                         OK (5.37s)
      +++ OK, passed 10000 tests; 63676 discarded.
      
      sizes (10000 in total):
      84.72% 1
      11.30% 0
       2.91% 2
       0.98% 3
       0.09% 4
    prop_genTypeCorrect:                                OK (13.63s)
      +++ OK, passed 10000 tests.
    prop_genWellTypedFullyApplied:                      OK (4.87s)
      +++ OK, passed 1000 tests.
    prop_findInstantiation:                             OK (1.52s)
      +++ OK, passed 10000 tests.
    prop_inhabited:                                     OK (6.21s)
      +++ OK, passed 3000 tests.
    prop_stats_numShrink:                               OK (3.81s)
      +++ OK, passed 40 tests.
      
      distribution | duplicates (40 in total):
      62%          | 0%
      22%          | 5%
      10%          | 10%
       2%          | 15%
       2%          | 20%
    prop_noTermShrinkLoops:                             OK (6.83s)
      +++ OK, passed 40 tests.
    prop_shrinkTermSound:                               FAIL
      *** Failed! Falsified (after 3 tests and 3 shrinks):
      ty,tm = ( integer -> d4_4
      , let data d4_4 | m2_2 where
              
        in error {integer -> d4_4} )
      [ E019: Error during PIR typechecking:
      Free type variable at  () :  d4
      , E019: Error during PIR typechecking:
      Free type variable at  () :  d4
      , E019: Error during PIR typechecking:
      Free type variable at  () :  d4 ]
      Use --quickcheck-replay=257544 to reproduce.
      Use -p '/prop_shrinkTermSound/' to rerun this test only.

1 out of 184 tests failed (102.05s)

Extricated from https://input-output.atlassian.net/browse/PLT-1333

[effectfully]

There was a test failure on prop_shrinkTermSound:

I can no longer reproduce it.

Closing in favor of #6084 6531.