[RISCV] Add DAG combine to fold (select C, (add X, Y), Y) -> (add (select C, X, 0), Y).

Similar for sub, or, and xor. These are all operations that have 0
as a neutral value. This is based on a similar tranform in InstCombine.

This allows us to remove some XVentanaCondOps patterns and
some code from DAGCombine for RISCVISD::SELECT_CC.

Reviewed By: asb

Differential Revision: https://reviews.llvm.org/D140465