[RISCV] Canonicalize vrot{l,r} to vrev8 when lowering shuffle as rotate

A rotate of 8 bits of an e16 vector in either direction is equivalent to a
byteswap, i.e. vrev8. There is a generic combine on ISD::ROT{L,R} to
canonicalize these rotations to byteswaps, but on fixed vectors they are
legalized before they have the chance to be combined. This patch teaches the
rotate vector_shuffle lowering to emit these rotations as byteswaps to match
the scalable vector behaviour.

Reviewed By: reames

Differential Revision: https://reviews.llvm.org/D158195