[RISCV] Separate single source and dual source lowering code [nfc]

The two single source cases aren't effected by the swap or select matching
as those are dual operand specific.  Similarly, a two source shuffle can't
be a rotate.

We can extend this idea for some of the shuffle types above, but some of
them are validly either single or dual source.  We don't want to loose that
and the code complexity of versioning early and having to repeat some shuffle
kinds doesn't (currently) seem worth it.