[CSOptimizer] Always prefer a disjunction with a single active choice

Disjunctions with a single element are sometimes introduced after
disfavoring, so we need to make sure that they are always preferred
during disjunction selection.

Author: xedin

lib/Sema/CSOptimizer.cpp

@@ -1431,6 +1431,9 @@ ConstraintSystem::selectDisjunction() {
         unsigned firstActive = first->countActiveNestedConstraints();
         unsigned secondActive = second->countActiveNestedConstraints();
 
+        if (firstActive == 1 || secondActive == 1)
+          return secondActive != 1;
+
         auto &[firstScore, firstFavoredChoices] = favorings[first];
         auto &[secondScore, secondFavoredChoices] = favorings[second];