[llvm] [AMDGPU] Add documentation files for GFX12. (PR #157151)

[Konstantin Zhuravlyov via llvm-commits]

kzhuravl wrote:

Also need to add to AMDGPUUsage around  https://github.com/llvm/llvm-project/blob/main/llvm/docs/AMDGPUUsage.rst?plain=1#L17716

So it is included in this section's table https://llvm.org/docs/AMDGPUUsage.html#instructions


https://github.com/llvm/llvm-project/pull/157151