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

[Matt Arsenault via llvm-commits]

arsenm 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.

The DAG is supposed to require LCSSA, it should get recomputed if broken later 

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