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

[James Y Knight via llvm-commits]

jyknight wrote:

OK -- well, the 1-line change to AtomicExpand LGTM.

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.

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