Rather than converting 3 signbits to bools and comparing them,
we can do bitwise logic on the whole vector and convert the
resulting sign bit to a bool at the end.
This is still a different algorithm than what we do in LegalizeDAG
through expandSADDOSSUBO. That algorithm needs to know that the
RHS of SSUBO is > 0, but that's costly when the type is split.
This "Node->getOpcode() == ISD::SADDO" is done several times in ExpandIntRes_SADDSUBO now (including a switch that can probably be reduced to an assert + compares).