infer nullity: treat instanceof as null check (IDEA-98153)

java/java-tests/testData/codeInsight/nullityinferrer/beforeParameterCheckedForInstanceof.java

@@ -0,0 +1,12 @@
+public class C {
+  private int x;
+
+  public boolean equals(Object object) {
+    if (object instanceof C) {
+      if (((C)object).x == x) {
+        return true;
+      }
+    }
+    return false;
+  }
+}