[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" 

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