[LegalizeIntegerTypes] Further improve ExpandIntRes_SADDSUBO for targets where...

[LegalizeIntegerTypes] Further improve ExpandIntRes_SADDSUBO for targets where SADDO/SSUBO aren't supported.

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.

Reviewed By: RKSimon

Differential Revision: https://reviews.llvm.org/D97325