Adds support for selecting the following instructions using GlobalISel:
- v_mad_mix/v_fma_mix
- v_mad_mixhi/v_fma_mixhi
- v_mad_mixlo/v_fma_mixlo
To select those instructions properly, some additional changes were
needed which impacted other tests as well.
Is this required? I am confused about whether you are now matching VOP3PMadMixMods in TableGen patterns, or only doing it from the C++ code in selectG_FMA.