[RISCV] Add simple DAG combine to pull xor with 1 through select_cc.

If we're selecting the result of two setccs that have been legalized
by introducing an xor with 1, we can pull the xor with 1 through the
select to enable more optimizations.

We could generalize this to other binary operators with identical
conditions, but those are usually caught before we legalize the select.

Reviewed By: asb

Differential Revision: https://reviews.llvm.org/D156144