[RISCV] Remove RISCVISD::BREV8 and use RISCVISD::GREV instead.

We already have an ISD opcode for the more general GREV/GREVI
instructon. We can just use it with the encoding that corresponds
to the behavior of brev8. This is similar to what we do for orc.b
where we use the GORC ISD opcode.