[Reducer] Improve simplification for reducing strategies (#15080)

This patch makes the existing reducing strategies produce smaller code.
Without this patch, unused private dispatches are left in the final
output along with other dead code.

compiler/src/iree/compiler/Reducer/Strategies/ReduceFlowDispatchOperandToResult.cpp

@@ -42,16 +42,18 @@ void mlir::iree_compiler::Reducer::reduceFlowDispatchOperandToResultDelta(
     return;
   }
 
-  // Replace all dispatch ops with random inputs.
+  // Replace all dispatch ops with the chosen operand.
   for (auto [result, operand] : resultToOperand) {
     result.replaceAllUsesWith(operand);
   }
 
-  // Simplify.
   PassManager pm(module.getContext());
-  pm.addPass(createCanonicalizerPass());
+  // Dead code eliminate the dispatch ops.
   pm.addPass(createCSEPass());
+  // Dead code eliminate the weights.
   pm.addPass(createSymbolDCEPass());
+  // Canonicalize the module.
+  pm.addPass(createCanonicalizerPass());
   if (failed(pm.run(module))) {
     return;
   }

compiler/src/iree/compiler/Reducer/Strategies/ReduceFlowDispatchResultBySplat.cpp

@@ -80,4 +80,17 @@ void mlir::iree_compiler::Reducer::reduceFlowDispatchResultBySplatDelta(
     // Erase the dispatch.
     dispatch.erase();
   }
+
+  PassManager pm(module.getContext());
+  // Dead code eliminate the dispatch ops.
+  pm.addPass(createCSEPass());
+  // Dead code eliminate globals.
+  pm.addPass(createSymbolDCEPass());
+  // Canonicalize so that the splats are fused with reshapes.
+  pm.addPass(createCanonicalizerPass());
+  // CSE again to de-duplicate splats.
+  pm.addPass(createCSEPass());
+  if (failed(pm.run(module))) {
+    return;
+  }
 }

compiler/src/iree/compiler/Reducer/Strategies/ReduceLinalgOnTensorsDelta.cpp

@@ -121,8 +121,12 @@ void mlir::iree_compiler::Reducer::reduceLinalgOnTensorsDelta(
   }
 
   PassManager pm(module.getContext());
-  pm.addPass(createCanonicalizerPass());
+  // Dead code eliminate.
   pm.addPass(createCSEPass());
+  // De-duplicate identical fills.
+  pm.addPass(createCanonicalizerPass());
+  // Remove dead globals.
+  pm.addPass(createSymbolDCEPass());
   if (failed(pm.run(module)))
     return;
 }