[llvm] [GlobalISel] Preserve volatile and ordered atomic stores of undef (PR #226265)

[Alex MacLean via llvm-commits]

AlexMaclean wrote:

@arsenm I had to regild some AMDGPU tests where volatile stores of undef are now preserved but otherwise no AMDGPU changes were needed. Change look reasonable and consistent with SDAG but please let me know if there are any concerns from the AMDGPU perspective.

https://github.com/llvm/llvm-project/pull/226265