TypeConstraint#union: smarter merge of supertypes

union("is X, Y; not SubX", "is SubX, Y") now produces "is X, Y"; before it was "is Y".
Fixes IDEA-184153 Smart-cast completion fails

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

@@ -0,0 +1,25 @@
+import java.math.BigInteger;
+import java.util.function.Function;
+
+class Test {
+    interface X {
+        void methodFromX();
+        void methodFromX2();
+    }
+
+    interface Y {
+        void methodFromY();
+        void methodFromY2();
+    }
+
+    interface Z extends X {}
+
+    void test(Object obj) {
+        if(obj instanceof Z && Math.random() > 0.5) {
+            return;
+        }
+        if(obj instanceof X && obj instanceof Y) {
+            obj.method<caret>
+        }
+    }
+}