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

[Matt Arsenault via llvm-commits]

arsenm wrote:

> On AMDGPU, this causes backend crashes. 

The combiner patch part still LGTM, though probably poorly tested. 

Based on the patch, the "backend crashes" ought to be selection failures, which are fine and ignorable. The AMDGPU patch is still not right, and I'd just drop it since full support for exotic pointer types is well beyond the scope of this PR 

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