[RISCV] Handle seteq/setne conditions for CZERO_NEZ/CZERO_EQZ during isel.

This removes selectSETCC and adds isel patterns for seteq/setne
conditions.

This removes the duplication of selectSETCC between lowering and
isel. This also gets some cases in xaluo.ll that we missed previously.

Reviewed By: wangpc

Differential Revision: https://reviews.llvm.org/D156250