TypeConstraint#union: fixed possible inconvertibility with inconsistent hierarchy

Fixes EA-123372 - NPE: AbstractStreamEx.lambda$foldLeft$

java/java-tests/testData/codeInsight/completion/normal/InconsistentHierarchyDfa_after.java

@@ -0,0 +1,14 @@
+import java.util.*;
+
+class Foo {
+    final class X {}
+    class Y extends X {}
+    class YY extends X {}
+    interface Z {};
+
+    void test(Object o) {
+        if((o instanceof Y && o instanceof Z) || (o instanceof YY && o instanceof Z)) {
+            o.hashCode()<caret>
+        }
+    }
+}