OMod was disabled if OpSel was enabled, but that restriction is more
specific than necessary. Any VOP3 with float operands can use OMod.
On GFX11, FMAC_F16_e64 can use op_sel.
Previously, SIFoldOperands and convertToThreeAddress were accidentally correct when
they reinterpreted the zero OMod operand on V_FMAC_F16_e64 as the OpSel operand on
V_FMA_F16_gfx9_e64. Now we explicitly add op_sel if required.
Can you precommit this part?