[DAG] Fold (abds x, y) -> (abdu x, y) iff both args are known positive

This is a generic DAG combine version of D151055 which recognizes when a signed ABDS can be safely replaced with a unsigned ABDU instruction if it is legal.

Alive2: https://alive2.llvm.org/ce/z/pb5BjG

Differential Revision: https://reviews.llvm.org/D153328