[llvm] [ConstraintElim] Use cond from header as upper bound on IV in exit BB. (PR #94610)

[Florian Hahn via llvm-commits]

fhahn wrote:

> LGTM if no compile-time impact.
> 
> Please double-check whether this actually fixes #90417 (with current phase ordering) and drop the line from the PR description if not.

Thanks, compile-time impact looks neutral. As @v01dXYZ pointed out, it the current version doesn't apply to https://github.com/llvm/llvm-project/issues/90417 with the current phase ordering. I removed the `Fixes...` and I'll add some phase ordering tests. We would also need to handle phis with multiple incoming values (a separate improvement which would probably be good to have anyways)

https://github.com/llvm/llvm-project/pull/94610