8280035: Use Class.isInstance instead of Class.isAssignableFrom where applicable

Reviewed-by: prr, rriggs

src/java.base/share/classes/java/lang/reflect/AccessibleObject.java

@@ -481,7 +481,7 @@ public class AccessibleObject implements AnnotatedElement {
             }
             // if this object is an instance member, the given object
             // must be a subclass of the declaring class of this reflected object
-            if (!declaringClass.isAssignableFrom(obj.getClass())) {
+            if (!declaringClass.isInstance(obj)) {
                 throw new IllegalArgumentException("object is not an instance of "
                                                    + declaringClass.getName());
             }