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

[Matt Arsenault via llvm-commits]

arsenm wrote:

> Is it legal to preserve aliasing metadata? What if we had a narrow rmw widened to a larger type where only the additional bits accessed aliased an unrelated access?

I would hope it does not matter - the extra high bits are put back exactly as they were 

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