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

[Matt Arsenault via llvm-commits]

arsenm wrote:

> I can’t speak for AMDGPU specifics, just that this code change is correct with respect to LangRef, and that LangRef’s model works for CHERI’s fat pointers.

Right, this currently avoids a verifier error and in theory should work. Whether the system as a whole works is an AMDGPU codegen issue for another time 

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