[llvm] [JumpThreading] Thread a common icmp across PHI nodes (PR #221125)

[Florian Hahn via llvm-commits]

https://github.com/fhahn commented:

I tried to reduce some of the cases I think are regressions. Each of the function in https://llvm.godbolt.org/z/EWxjWTGr5 regresses with the patch AFAICT

https://github.com/llvm/llvm-project/pull/221125