[AMDGPU] Shrink MAD/FMA to MADAK/MADMK/FMAAK/FMAMK on GFX10

On GFX10 VOP3 instructions can have a literal operand, so the conversion
from VOP3 MAD/FMA to VOP2 MADAK/MADMK/FMAAK/FMAMK will not happen in
SIFoldOperands. The only benefit of the VOP2 form is code size, so do it
in SIShrinkInstructions instead.

Differential Revision: https://reviews.llvm.org/D125567