[clang] [llvm] [AMDGPU] Add builtins for FMA mix instructions (PR #214814)

[Matt Arsenault via llvm-commits]

https://github.com/arsenm requested changes to this pull request.

These do not require user facing interface, and we should excluively pattern match from the primitives 

https://github.com/llvm/llvm-project/pull/214814