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

[Valery Pykhtin via llvm-commits]

vpykhtin wrote:

Used `EliminateDuplicatePHINodes` because it's much faster.

Turned on optimizations by default, got some failures but they're mostly reorder of phi's. 

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