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

[Sjoerd Meijer via llvm-commits]

sjoerdmeijer wrote:

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?

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