[llvm] [AMDGPU][True16][GlobalISel] Update True16 handling for FMA_MIX instructions in GlobalISel (PR #205431)

[Petar Avramovic via llvm-commits]

https://github.com/petar-avramovic commented:

somewhat acceptable way to do this would be to have more complicated ComplexRendererFns in particular the return part that usually just does addReg, and build the reg-sequence there under some condition.
Check how copyToVGPRIfSrcFolded is used

https://github.com/llvm/llvm-project/pull/205431