[AMDGPU] Move V_FMA_MIX pattern matching into tablegen. NFC

The matching for V_FMA_MIX was partially implemented with a C++
matcher (for fmas with 32 bit results and 16 bit inputs) and partially
in tablegen (for fmas with 16 bit results). Move the C++ matcher logic
into tablegen to make this more consistent and so we can remove the
duplication between SDAG and GISel.

Differential Revision: https://reviews.llvm.org/D144612