[llvm] [llvm][UnifyLoopExits] Avoid optimization if no exit block is found (PR #165343)

[Dmitry Sidorov via llvm-commits]

MrSidims wrote:

@Saldivarcher I'm fine with the PR (and also interested in merging it). Yet I'd like to hear from @ssahasra as he is more familiar with this part of the code base, so lets have some graceful period before the merge.

https://github.com/llvm/llvm-project/pull/165343