[DAGCombiner][RISCV] Canonicalize (bswap(bitreverse(x))->bitreverse(bswap(x)).

If the bitreverse gets expanded, it will introduce a new bswap. By
putting a bswap before the bitreverse, we can ensure it gets cancelled
out when this happens.

Reviewed By: RKSimon

Differential Revision: https://reviews.llvm.org/D118012