[llvm] AtomicExpand: Fix creating invalid ptrmask for fat pointers (PR #94955)

[Matt Arsenault via llvm-commits]

arsenm wrote:

> I'll defer to AMDGPU experts about the AMDGPU tests. If those are controversial or need more work, I'd also be OK with just taking the 1-line patch without those tests.

Even if the codegen were wrong here, step 1 would be to precommit these showing it's wrong anyway 



https://github.com/llvm/llvm-project/pull/94955