[AMDGPU] Fix delay alu for VOPD with src2acc

V_FMAC_F32 and V_DOT2C_F32_F16 have a dummy src2 operand tied to vdst to
inform passes that the instructions read the dst operand. The VOPD
versions of these instructions lacked the dummy operand, which was a
problem for inserting s_delay_alu.
Introduce the dummy src2 operand on the VOPD versions, and fix the VOPD operand
tracking logic to account for it.

Reviewed By: dp

Differential Revision: https://reviews.llvm.org/D136629