Avoid boxing loop variables that are only mutated in initializer or update

tests/compiler/dart2js/js_backend_cps_ir_closures_test.dart

 // Copyright (c) 2014, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 // VMOptions=-DUSE_CPS_IR=true
 
 // Tests of closures.
 
 library closures_test;
 
 import 'js_backend_cps_ir.dart';
@@ skipping 84 matching lines @@
 main() {
   var a;
   for (var i=0; i<10; i++) {
     a = () => i;
   }
   print(a());
 }
 """,
 r"""
 function() {
-  var a, box_0, i, v0, box_01, v1;
+  var a, i, v0, v1;
   a = null;
-  box_0 = {};
-  box_0.i_0 = 0;
+  i = 0;
   while (true) {
-    i = box_0.i_0;
     v0 = 10;
     if (P.identical(J.getInterceptor(i).$lt(i, v0), true)) {
-      a = new V.main_closure(box_0);
-      box_01 = {};
-      box_01.i_0 = box_0.i_0;
-      i = box_01.i_0;
+      a = new V.main_closure(i);
       v1 = 1;
-      box_01.i_0 = J.getInterceptor(i).$add(i, v1);
-      box_0 = box_01;
+      i = J.getInterceptor(i).$add(i, v1);
     } else {
       P.print(a.call$0());
       return null;
     }
   }
 }"""),
 ];
 
 void main() {
   runTests(tests);
 }