[turbofan] Relax a too-strict dcheck.

test/mjsunit/compiler/regress-v8-5573.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: --allow-natives-syntax
 
+var global = true;
+global = false;
+
 function f() {
-  return typeof null === "object";
-};
+  return !global;
+}
 
 %OptimizeFunctionOnNextCall(f);
 assertTrue(f());