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

[Matt Arsenault via llvm-commits]

arsenm wrote:

The intention is certainly to handle anything exotic, but the current SelectionDAGBuilder implementation just inserts an extend with and, which a target would need some way of overriding.

I also suspect this extension code in the DAGBuilder is dead code. I believe the verifier check that the mask type matches the index type is new since I last looked at this code 

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