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

[Jessica Clarke via llvm-commits]

jrtc27 wrote:

It’s written to the IR model that the address is in the low bits, because that’s a sensible thing to do. Even if your hardware doesn’t actually do that you can still model it in the IR as doing so and move bits around as needed in the backend. And if someone really wants to expose hardware that doesn’t do that to the IR they can be the one to update LangRef to be more general. Fundamentally ptrmask is about masking the address, it’s just written that way because it’s simplifying.

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