[turbofan] Better handling of empty type in simplified lowering.

test/mjsunit/compiler/regress-625558.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
 
+for (var global = 0; global <= 256; global++) { }
+
 function f() {
-  return typeof null === "object";
-};
+  global = "luft";
+  global += ++global;
+}
 
-%OptimizeFunctionOnNextCall(f);
-assertTrue(f());
+f();