[RISCV] Update combine-vmv.ll to have more foldable cases. NFC
A lot of cases have differing AVLs which aren't foldable, update them so the peephole triggers on them and add explicit cases for non-foldable AVLs.
Also rename it to vmv.v.v-peephole.ll since it's not actually a DAG combine.
And remove a TODO, it's correct to fold if the two passthrus are the same.