Some where clause lowering simplifications

[compiler-errors]

Rename PredicateFilter::SelfThatDefines to PredicateFilter::SelfTraitThatDefines to make it clear that it's only concerned with converting traits, and make it do a bit less work when converting bounds.

Also, make the predicate filter matching in probe_ty_param_bounds_in_generics explicit, and simply the args it receives a bit.

[rustbot]

r? @petrochenkov

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[rustbot]

HIR ty lowering was modified

cc @fmease

[petrochenkov]

Are there any observable effects from the second commit?
Or it's just a refactoring? (Due to the logic at the end of fn type_param_predicates?)
I'm not familiar with this code.

r=me after the explanation.
@rustbot author

[compiler-errors]

No, this should not have observable changes.

SelfTraitThatDefines is a filter that is concerned with gathering trait predicates of the form ParamTy: Trait for a given ParamTy. We did this previously somewhat haphazardly, both when lowering the bounds, and then re-filtering the bounds again after they've been collected, since the existing predicate filter handling was too lax.

Let me add a debug assertion back that I removed to validate that any further refactorings to the PredicateFilters doesn't accidentally reintroduce this behavior.

[compiler-errors]

forgot to flip back to ready:

@rustbot ready

[bors]

☔ The latest upstream changes (presumably #132377) made this pull request unmergeable. Please resolve the merge conflicts.