[Power9] Remove the PPCISD::XXREVERSE as it has completely the same semantics of ISD::BSWAP

The custom node PPCISD::XXREVERSE has completely the same semantics of generic node ISD::BSWAP.
We need to clean up it as we have the combine rules for bswap in the base class, while nothing for xxreverse.

Differential Revision: https://reviews.llvm.org/D70657