[llvm] [SSAUpdaterBulk] Add PHI simplification pass. (PR #132004)

[Valery Pykhtin via llvm-commits]

vpykhtin wrote:

Hi @nikic, I believe this is ready for submission in its current state. I chose to keep this code as a separate 'pass' rather than integrating it directly into RewriteAllUses for two reasons:

1. Identifying an equivalent PHI in `RewriteAllUses `and performing the subsequent RAUW would require using `TrackingVH `in `BBInfo`.

2. We can rename `RewriteAndSimplifyAllUses `to `RewriteAllUses `in a separate patch.

https://github.com/llvm/llvm-project/pull/132004