[RISCV] Commute Src in foldVMV_V_V (#170536)
In #156499 we taught the vmerge peephole to commute operands so that the
passthru operands lined up. We can do the same for the vmv.v.v peephole,
which allows us fold more vmv.v.vs away.
This is needed to prevent a regression in an upcoming patch that adds a
combine for vmerge.vvm to vmv.v.v.
