[LegalizeTypes] Add a special case for (add X, 1) to ExpandIntRes_ADDSUB.

On targets without ADDCARRY or ADDE, we need to emit a separate
SETCC to determine carry from the low half to the high half. Usually
we do (setult Lo, LHSLo). If RHSLo is 1 we can instead do (seteq Lo, 0).
This can reduce the live range of LHSLo.