AMDGPU/GlobalISel: Fix crash after mad/fma_mix fails selection

When selectVOP3PMadMixModsImpl fails, it can still create new copy instr
via selectVOP3ModsImpl. When selectG_FMA_FMAD gives up, new copy instr
will remain dead but will not be automatically removed.
InstructionSelect does not check if instructions created during selection
are dead.
Such dead copy doesn't have register class on dst operand and causes crash.
Fix is to build copy when operands are being added to selected instruction.

Differential Revision: https://reviews.llvm.org/D138044