[RISCV] Return false from isShuffleMaskLegal except for splats.

We don't support any other shuffles currently.

This changes the bswap/bitreverse tests that check for this in
their expansion code. Previously we expanded a byte swapping
shuffle through memory. Now we're scalarizing and doing bit
operations on scalars to swap bytes.

In the future we can probably use vrgather.vx to do a byte swap
shuffle.