[llvm] [AMDGPU] Select flat GVS atomics on gfx1250 (PR #149554)

[Stanislav Mekhanoshin via llvm-commits]

rampitec wrote:

@mariusz-sikora-at-amd this will also have merge conflicts downstream, particularly it adds CPol operand to `SelectGlobalSAddr()`, but not all features of these functions are upstreamed yet. Plus test checks a largely different because of the remaining code changes are not there yet. However, selection of atomics is already correct.

https://github.com/llvm/llvm-project/pull/149554