[RISCV] Improve RV64 codegen for i32 ISD::SADDO when RHS is constant.

This uses the same sequence we get from LegalizeDAG for i32 on RV32, but modified
to use W instructions.

When the RHS is constant one of the setccs simplifies to a constant and the xor will either
be an xori with 1 or get removed.

When the RHS is not a constant it was not an obvious improvement and it was a regression
when used with a branch. So I've restricted to the constant case.

Reviewed By: reames

Differential Revision: https://reviews.llvm.org/D150135