[llvm] [Utils][UnifyLoopExits] Avoid costly updates if nothing changed (PR #129179)

[Johannes Doerfert via llvm-commits]

jdoerfert wrote:

FWIW, the backend seems to be impacted by the missing LCSSA, see we could enforce that if we want to, and it's likely cheaper than what was done before.

https://github.com/llvm/llvm-project/pull/129179