[llvm] [AMDGPU] Use wavefront scope for single-wave workgroup synchronization (PR #187673)

[Barbara Mitic via llvm-commits]

barbara-amd wrote:

Addressed all three: demotion isn’t GFX12-only - tests now include several subtargets across cache-control paths; non-relaxed WG atomics + fences go through SIMemOpInfo scope demotion with new tests; AMDGPUUsage.rst memory-model section documents WG scope ≡ wavefront when the work-group is a single wavefront.

https://github.com/llvm/llvm-project/pull/187673