[llvm] [MemProf][PGO] Prevent dropping of profile metadata during optimization (PR #121359)

[Teresa Johnson via llvm-commits]

teresajohnson wrote:

@nikic curious for your thoughts on combineMetadata - it seems really fragile to new types of metadata, or new invocations such as in the case of the MemCpyOpt callsite, making it very easy to silently lose metadata (even !prof metadata was affected in one case I saw in real code). Is there a better way to do this?

https://github.com/llvm/llvm-project/pull/121359