[RISCV] Add DAG combine to pull xor with 1 through select idiom that uses czero_eqz/nez.

If we are selecting between two setccs that need to be legalized
with xor, the select will be legalized first. Detect this pattern
so we can pull the xor through to expose it to additional
optimizations.

We could generalize this to other operations, but those normally
get handled in DAG combine before select legalization.

Reviewed By: asb

Differential Revision: https://reviews.llvm.org/D156159