dart2js cps: Generate increment and compound operators.

tests/compiler/dart2js/js_backend_cps_ir_gvn_test.dart

 // Copyright (c) 2015, 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.
 
 // Test that the GVN optimization pass works as expected.
 
 library gvn_tests;
 
 import 'js_backend_cps_ir.dart';
 
@@ skipping 35 matching lines @@
   var x3 = new Leaf(20);
   var x4 = new Leaf(-10);
   var y1 = new Branch(x1, x2);
   var y2 = new Branch(x3, x4);
   var z  = new Root(y1, y2);
   print(foo(z, [1,2,3,4,5,6,7,8,9,10]));
 }
 """,r"""
 function() {
   var v0 = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10], sum = 0, k = 0, i, v1;
-  for (; k < 10; sum = sum + (i + v0[v1]), k = k + 1) {
+  for (; k < 10; sum += i + v0[v1], ++k) {
     i = 1 + 20;
     v1 = i * (10 + -10);
     if (v1 < 0 || v1 >= 10)
       return H.ioore(v0, v1);
   }
   v0 = H.S(sum);
   if (typeof dartPrint == "function")
     dartPrint(v0);
   else if (typeof console == "object" && typeof console.log != "undefined")
     console.log(v0);
   else if (!(typeof window == "object")) {
     if (!(typeof print == "function"))
       throw "Unable to print message: " + String(v0);
     print(v0);
   }
 }"""),
 ];
 
 void main() {
   runTests(tests);
 }