[AMDGPU] Remove old operand from VOPC DPP

For most DPP instructions, the old operand stores the value that was in
the current lane before the DPP operation, and is tied to the
destination. For VOPC DPP, this is unnecessary and incorrect.

There appears to have been a latent bug related to D122737 with
SIInstrInfo::isOperandLegal. If you checked if a register operand was legal
when the InstructionDesc expected an immediate, it reported that is valid.
Its fix is necessary for and tested in this patch.

Reviewed By: foad, rampitec

Differential Revision: https://reviews.llvm.org/D130040