[llvm] [GlobalISel] Preserve volatile and atomic undef/poison stores (PR #200099)

[Matt Arsenault via llvm-commits]

https://github.com/arsenm commented:

The store combiner change looks fine, but the AMDGPU changes are unrelated. Falling short of properly implementing these as directly handled types, I'd rather leave unhandled address-spaces as-is as a todo 

https://github.com/llvm/llvm-project/pull/200099