[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)).