DFA: instanceof for unknown variable still may perform declared type check

Fixes IDEA-191374 DFA: "condition `x instanceof X` can be replaced with a null check" is not reported

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

@@ -0,0 +1,12 @@
+// IDEA-191374
+class B {}
+class A {
+  private B b = new B();
+  void foo() {
+    String s = b != null ? b.toString() : ""; //comment this line and the error will appear
+
+    if (<warning descr="Condition 'b instanceof B' is redundant and can be replaced with a null check">b instanceof B</warning>) {  // no error reported here
+      System.out.println(b);
+    }
+  }
+}