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

[Pierre van Houtryve via llvm-commits]

https://github.com/Pierre-vh commented:

Thanks, that looks better. A few nits from me but other than that it's good.
I'd like some other memory model folks to look at this, but if no one comments I'll approve in the following days. @ssahasra @ritter-x2a 

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