[llvm] [PHIElimination] Reuse existing COPY in predecessor basic block (PR #131837)

[Florian Hahn via llvm-commits]

https://github.com/fhahn commented:

> We also see a miscompile caused by this. We don't have a reproducer yet, but it looks like there are quite a few already.
> 
> Do we need to revert this?

Would probably make sense to go back to a known-good state for now, but please make sure you share the additional reproducer once it is ready

https://github.com/llvm/llvm-project/pull/131837