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

[Pierre van Houtryve via llvm-commits]

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

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

Agreed, I thought about doing it here but I feel like it's better done as a separate task. There needs to be a discussion on what "granularity" we want the split to be done.

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