Auto merge of #99232 - lcnr:no-bound-vars-check, r=jackh726

Auto merge of #99232 - lcnr:no-bound-vars-check, r=jackh726

`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`