[llvm] [AtomicExpand] Preserve AA metadata when rewriting atomic operations (PR #206418)

[Matt Arsenault via llvm-commits]

arsenm wrote:

> hmm, this is a bit hand-wavy but I think it might be legal to preserve the metadata for widened atomics. The widened version reads some extra bytes and writes the same bytes atomically, it has the same semantics as the original cmpxchg. So the value that we read and write for those extra bytes doesn't have any affect on the program, it's as if we didn't touch them at all.

This is essentially my thinking, it's just a question of the specifics of the spooky provenance metadata rules

https://github.com/llvm/llvm-project/pull/206418