[RISCV] Fold (add (select lhs, rhs, cc, 0, y), x) -> (select lhs, rhs, cc, x, (add x, y))

Similar for sub except sub isn't commutative.

Modify the existing and/or/xor folds to also work on ISD::SELECT
and not just RISCVISD::SELECT_CC. This is needed to make sure
we do this transform before type legalization turns i32 add/sub
into add/sub+sign_extend_inreg on RV64. If we don't do this before
that, the sign_extend_inreg will still be after the select.

Reviewed By: frasercrmck

Differential Revision: https://reviews.llvm.org/D107603