[turbofan] Implement top-level lookup slot declaration.

test/mjsunit/ignition/regress-599001-verifyheap.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: --verify-heap --expose-gc
+// Flags: --ignition --verify-heap --expose-gc
 
 // Tests that verify heap works for BytecodeArrays in the large object space.
 
 // Creates a list of variable declarations and calls it through eval to
 // generate a large BytecodeArray.
 var s = "";
 for (var i = 0; i < 65535; i++) {
   s += ("var a" + i + ";");
 }
 
 (function() { eval(s); })();
 gc();