[SCEV] Fix finite loop non-strict predicate simplification (PR60944)

There are a number of issues with the current code for converting
ule -> ult (etc) predicates for comparisons controlling finite loops:

* It sets nowrap flags, which may only hold for that particular
  comparison, not globally. (PR60944)
* It doesn't check that the RHS is invariant. (I'm not sure this
  can cause practical issues independently of the previous point.)
* It runs before simplifications that may be more profitable. (PR54191)

This patch moves the handling for this into computeExitLimitFromICmp(),
because it is somewhat tightly coupled with assumptions in that code,
and addresses the aforementioned issues.

Fixes https://github.com/llvm/llvm-project/issues/60944.
Fixes https://github.com/llvm/llvm-project/issues/54191.

Differential Revision: https://reviews.llvm.org/D145510