…const-prop

Fixes rust-lang#68264

Previously, I attempted to use
`substitute_normalize_and_test_predicates` to detect unsatisfiable
bounds. Unfortunately, since const-prop runs in a generic environment
(we don't have any of the function's generic parameters substituted),
this could lead to cycle errors when attempting to normalize predicates.

This check is replaced with a more precise check. We now only call
`normalize_and_test_predicates` on predicates that have the possibility
of being proved unsatisfiable - that is, predicates that don't depend
on anything local to the function (e.g. generic parameters). This
ensures that we don't hit cycle errors when we normalize said
predicates, while still ensuring that we detect unsatisfiable
predicates.

Co-Authored-By: Oliver Scherer <github35764891676564198441@oli-obk.de>