[llvm] AtomicExpand: Preserve metadata when expanding partword RMW (PR #89769)

[Matt Arsenault via llvm-commits]

arsenm wrote:

> > Is it really a good idea to blindly preserve all unknown metadata?
> > ISTM we have no way of knowing if the metadata will be valid on the modified instruction or not. Maybe better to preserve only known-to-be-OK metadata?
> 
> The cases I really need to preserve are target string metadata, i.e. "unknown"

So options are 1. hardcode the amdgpu prefixed metadata, or 2. promote the AMDGPU metadata to one of the builtin recognized metadata types? 

https://github.com/llvm/llvm-project/pull/89769