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

[Alexander Richardson via llvm-commits]

https://github.com/arichardson approved this pull request.

The change looks good to me and matches my understanding of what _should_ be more explicit in the LangRef.

A while ago I tried to make the index<->address relationship more explicit (https://reviews.llvm.org/D135158), but there was no consensus to land that.

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