[llvm] [IR] Avoid self-referencing values caused by PHI node removal (PR #129501)

[Robert Imschweiler via llvm-commits]

ro-i wrote:

I added another implementation. This checks much more aggressively for blocks that become unreachable during pass execution. Consequently, I had to update a few of the existing tests because their checks contained unreachable blocks, which get deleted now by the pass. (If you agree that this is a viable solution, I may further need to adapt some of these tests because right now, they may not check for their actual test case anymore.

https://github.com/llvm/llvm-project/pull/129501