[llvm] [SCEV] Add PHI SCEV verification to ensure availability at loop entry (PR #127550)

[Aleksandr Popov via llvm-commits]

aleks-tmb wrote:

@nikic @fhahn Hi,

Here’s another case of missing SCEV invalidation, which leads to incorrect availability at the loop entry: https://github.com/llvm/llvm-project/issues/130930

The reason the existing loop and block disposition verification doesn't catch this issue is that disposition caches are cleared inside the `loop-simplifycfg` pass. 
However, when `isAvailableAtLoopEntry` is called, it recomputes the disposition for the cached SCEV and exposes the issue.

https://github.com/llvm/llvm-project/pull/127550