don't let null checks affect 'unchecked cast' analysis

java/java-tests/testData/inspection/dataFlow/TryWithResources.java

@@ -36,7 +36,7 @@ class Test {
       System.out.println(r);
     }
     catch (Exception e) {
-      if (e instanceof ResourcefulException1) {
+      if (<warning descr="Condition 'e instanceof ResourcefulException1' is always 'false'">e instanceof ResourcefulException1</warning>) {
         System.out.println("1");
       }
       else if (e instanceof ResourcefulException2) {

java/java-tests/testData/inspection/dataFlow/fixture/NullCheckDoesntAffectUncheckedCast.java

@@ -0,0 +1,15 @@
+public class DataFlowBug {
+
+  public int add2(Object left, Object right) {
+    if (left != null && !(left instanceof String)) {
+      return ((<warning descr="Casting 'left' to 'String' may produce 'java.lang.ClassCastException'">String</warning>) left).length();
+
+    }
+    if (!(right instanceof String)) {
+      return ((<warning descr="Casting 'right' to 'String' may produce 'java.lang.ClassCastException'">String</warning>) right).length();
+
+    }
+    return 2;
+  }
+
+}