Remove incorrect optimization in getUnionConstraintOfIntersection

Author: ahejlsberg

src/compiler/checker.ts

@@ -7598,11 +7598,6 @@ namespace ts {
                         constraint = getConstraintOfType(constraint);
                     }
                     if (constraint) {
-                        // A constraint that isn't a union type implies that the final type would be a non-union
-                        // type as well. Since non-union constraints are of no interest, we can exit here.
-                        if (!(constraint.flags & TypeFlags.Union)) {
-                            return undefined;
-                        }
                         constraints = append(constraints, constraint);
                     }
                 }