[llvm] [AMDGPU][True16][CodeGen] true16 codegen for MadFmaMixPat (PR #124892)

[Joe Nash via llvm-commits]

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

It is clearly possible to improve the code quality in the True16 flow. I expect some of this to come from https://github.com/llvm/llvm-project/pull/103366 . Another thing we could do is implement new True16 pseduo instructions for fma_mix type instructions. I am just noting that as a possibility, it is not needed at this time.
The Fake16 path is unchanged, as desired.
LGTM.

https://github.com/llvm/llvm-project/pull/124892