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`replace_bound_vars` fast path: check predicates, don't check consts split out from #98900 `ty::Const` doesn't have precomputed type flags, so computing `has_vars_bound_at_or_above` for constants requires us to visit the const and its contained types and constants. A noop fold should be pretty much equally as fast so removing it prevents us from walking the constant twice in case it contains bound vars. r? `@jackh726`
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