[llvm] [LV] Forget LCSSA phi with new pred before other SCEV invalidation. (PR #119897)

[Florian Hahn via llvm-commits]

fhahn wrote:

> Looking at https://llvm.godbolt.org/z/xdWvqbhaj, `%add.lcssa` has SCEV `({0,+,1}<nuw><nsw><%loop.1> + %load)` with exit value `(100 + %load)`. Invalidating `%add.lcssa` should find the invalidation roots `{0,+,1}<nuw><nsw><%loop.1>` and `%load`. Invalidating `%load` should invalidate both the exit value and the BECount using it.

Had another look and it seems the issue was that we invalidated the SCEV for the phi before `forgetLcssaPhiWithNewPredecessor` could apply its specific invalidation logic. Updated the patch to first run `forgetLcssaPhiWithNewPredecessor`, then the other SCEV invalidation.

https://github.com/llvm/llvm-project/pull/119897