[SelectionDAG][RISCV] Teach getNode to fold bswap(bswap(x))->x.

This can show up during when bitreverse is expanded to bswap and
swap of bits within a byte. If the input is already a bswap, we
should cancel them out before we further transform them in a way
that makes it harder to see the redundancy.

Reviewed By: RKSimon

Differential Revision: https://reviews.llvm.org/D118007