[AMDGPU] Fix an assertion failure when folding into src2 of V_FMAC_F16

D139469 "[AMDGPU] Enable OMod on more VOP3 instructions" caused an
assertion failure when trying to fold into src2 of V_FMAC_F16. It would
temporarily convert the instruction to V_FMA_F16_gfx9 and add an opsel
operand, but if the fold still failed then it would forget to remove the
opsel operand.

Differential Revision: https://reviews.llvm.org/D144558