Generalize all representations when reconfiguring a property of a strict Function subclass.

test/mjsunit/regress/regress-crbug-575080.js

+// Copyright 2016 the V8 project authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+// Flags: --es-staging
+
+class A extends Function {
+  constructor(...args) {
+    super(...args);
+    this.a = 42;
+    this.d = 4.2;
+    this.o = 0;
+  }
+}
+var obj = new A("'use strict';");
+obj.o = 0.1;