[llvm] Reapply "[NFC][AMDGPU] Introduce AMDGPU::FlatVariant enum to replace uint64_t FLAT discriminant" (PR #200431)

[Shilei Tian via llvm-commits]

https://github.com/shiltian approved this pull request.

This looks good to me at it current form, but a thought: maybe directly merge this into `AMDGPUAS`, make them pretty large numbers, and then we can almost treat them as dedicated virtual AS.

https://github.com/llvm/llvm-project/pull/200431