Fix clobbered register when setting this_function variable.

test/mjsunit/regress/regress-crbug-498022.js

 // Copyright 2015 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: --harmony-classes --no-lazy
+
+// Flags: --debug-code --nouse-gvn
+
 "use strict";
 class Base {
 }
-class Subclass extends Base {
+class Derived extends Base {
   constructor() {
-      this.prp1 = 3;
+    eval();
   }
 }
-function __f_1(){
-}
+assertThrows("new Derived()", ReferenceError);