[RISCV] Add tests that do a bitreverse before or after a bswap. NFC

We don't optimize this as well as we could. Bitreverse is always
expanded to bswap and a shift/and/or sequence to swap bits within a
byte. The newly created bswap will either becomes a shift/and/or
sequence or rev8 instruction. We don't always realize the bswap is
redundant with another bswap before or after the bitreverse.

Found while thinking about the brev8 instruction from the
Cryptography extension. It's equivalent to bswap(bitreverse(x)) or
bitreverse(bswap(x)).