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

[Michael Kruse via llvm-commits]

Meinersbur wrote:

I didn't see that its throug the actions of JumpThreading. Where exactly does crash occur. Within DeleteDeadBlock? Why does it try to fix dead BBs? At the verifier afterwards? After InstCombine on it?

Simplest fix would be to not to try to cleanup unreachable code in JumpThreading. Just let ADCE/SimplifyCFG handle it. Or call EliminateUnreachableBlocks at the end. That would even be cheaper than `DeleteDeadBlocks` since each invocation calls `detachDeadBlocks` which also iterates over all BBs.

There is comment saying: 
> We must remove BB to prevent invalid IR.
Which is AFAIK wrong. Unreachable code in itself is not invalid IR. 

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