Since value types are not cloneable, JVM_Clone can enforce the check
rather than doing an assert. javac does not allow Object::clone be called from a value type but it could be called via reflection/method handle or generated bytecode.

  // Check if class of obj supports the Cloneable interface.
  // All arrays are considered to be cloneable (See JLS 20.1.5).
  // All j.l.r.Reference classes are considered non-cloneable.
  if (!klass->is_cloneable() ||
      (klass->is_instance_klass() &&
       InstanceKlass::cast(klass)->reference_type() != REF_NONE)) {
    ResourceMark rm(THREAD);
    THROW_MSG_0(vmSymbols::java_lang_CloneNotSupportedException(), klass->external_name());
  }
  assert(!EnableValhalla || !obj->klass()->is_value(), "Clone disallowed on value type");