[RISCV] Combine GREVI sequences

This combine step performs the following type of transformation:

    rev.p a0, a0   # grevi a0, a0, 0b01
    rev2.n a0, a0  # grevi a0, a0, 0b10
    -->
    rev.n a0, a0   # grevi a0, a0, 0b11

Reviewed By: craig.topper

Differential Revision: https://reviews.llvm.org/D91877