[llvm] [SCEV][LV] Invalidate LCSSA exit phis more thoroughly (PR #69909)

[Florian Hahn via llvm-commits]

https://github.com/fhahn commented:

Thanks for extending the invalidation. 

I'm wondering if expressions other than SCEVUnknown could also cause issues? IIUC any operand from the SCEV expression for the phi could be used outside and wouldn't those uses need invalidating as well? They might not cause crashes via `isAvailableAtLoopEntry`, could they cause slightly incorrect BECounts?

https://github.com/llvm/llvm-project/pull/69909