[DAG] Add computeOverflowForSignedAdd and computeOverflowForAdd wrapper

Add basic computeOverflowForSignedAdd helper to recognise that sadd overflow can't occur if both operands have more that one sign bit.

Add computeOverflowForAdd wrapper that calls computeOverflowForSignedAdd/computeOverflowForUnsignedAdd depending on the IsSigned argument, and use this in DAGCombiner::visitADDO