[AMDGPU] Disentangle MFMA handling in convertToThreeAddress. NFC.

Move MFMA handling to the top of convertToThreeAddress and pull
IsF16 calculation out of the switch. I think this makes it clearer
exactly which mac/fmac opcodes are handled, since they are now
listed in the switch with minimal extra clutter.

Differential Revision: https://reviews.llvm.org/D120703