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

[Matt Arsenault via llvm-commits]

arsenm wrote:

> For fat pointers, the type is more complicated. You have to consider the address part and the other components.

Right, that's supposed to expressed by the index type. Looking at the codegen for p7/p8 fat pointers, I think this actually works as expected 

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