[llvm] Reapply [Metadata] Preserve MD_prof when merging instructions when one is missing. (PR #135418)

[Nikita Popov via llvm-commits]

nikic wrote:

> Also add a legality check when merging prof metadata based on instruction type. Without this check GVN PRE optimizations result in prof metadata on phi nodes which break the module verifier.

Can you please explain in more detail what happens? This sounds suspicious.

https://github.com/llvm/llvm-project/pull/135418