[llvm] AMDGPU: Undo atomicrmw add/sub/xor 0 -> atomicrmw or canonicalization (PR #87533)

[Matt Arsenault via llvm-commits]

arsenm wrote:

> should we do this for other synch scopes? Or revisit it after we add metadata for no-fine-grained and no-remote?

I wanted to stay as targeted as possible. There will be an interaction with the new metadata, but it shouldn't be necessary outside of system scope 



https://github.com/llvm/llvm-project/pull/87533