[llvm] [Doc][AMDGPU] Add barrier execution & memory model (PR #170447)

[Nicolai Hähnle via llvm-commits]

https://github.com/nhaehnle commented:

Side note: It feels like it may be time to split up AMDGPUUsage.rst. It's become a bit unwieldy.

Overall, I really like this.

https://github.com/llvm/llvm-project/pull/170447