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

[Jessica Clarke via llvm-commits]

jrtc27 wrote:

Anyway, I encourage people in this thread to go and read LangRef (https://llvm.org/docs/LangRef.html#llvm-ptrmask-intrinsic), which is clear on the matter

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