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

[Krzysztof Drewniak via llvm-commits]

krzysz00 wrote:

@arsenm  That being said, this gets much simpler if we say "the base part of a ptr addrspace(7)'s buffer descriptor must be word-aligned on pain of UB" by analogy with image descriptors

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