Fix typeof optimization for undetectable

test/mjsunit/regress/regress-5972.js

+// Copyright 2017 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: --allow-natives-syntax
+
+var undetectable = %GetUndetectable();
+
+function foo(a) {
+  const o = a ? foo : undetectable;
+  return typeof o === 'function';
+}
+
+assertFalse(foo(false));
+assertFalse(foo(false));
+%OptimizeFunctionOnNextCall(foo);
+assertFalse(foo(false));