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

[Mikael Holmén via llvm-commits]

mikaelholmen wrote:

Also note that there are unsolved debug-info related problems with the patch as mentioned here:
 https://github.com/llvm/llvm-project/pull/131837#discussion_r2174969801
(not sure if anyone actually sees the new comments there, that's why I mention it again here in the conversation)

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