On targets that lack ADDCARRY support we split a wide uaddo into
an ADD and a SETCC that both need to be split.
For (uaddo X, 1) we can observe that when the add overflows the result
will be 0. We can emit (seteq (or Lo, Hi), 0) to detect this.
This improves D142071.
There is an alternative here. We could use either ~(lo(X) & hi(X)) == 0 or
(lo(X) & hi(X)) == -1 before the addition. That would be closer to the
code before D142071.