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.
The uaddo case below uses a comparison of two add flavors. Does the same notion work here for the non-constant case?