cps-ir: Merge variables based on set-based liveness and graph coloring.

tests/compiler/dart2js/js_backend_cps_ir_interceptors_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 interceptors.
 
 library interceptors_tests;
 
 import 'js_backend_cps_ir.dart';
@@ skipping 19 matching lines @@
   print(l.length);
   for (int i  = 0; i < l.length; i++) {
     var x = l[i];
     for (int j = 0; j < x.length; j++) {
       print(x[j]);
     }
   }
 }""",
 r"""
 function() {
-  var l, i, v0, x, j, v1;
+  var l, i, v0, x, j;
   l = ["hest", ["h", "e", "s", "t"]];
   P.print(J.getInterceptor$as(l).get$length(l));
   i = 0;
   L0:
     while (true) {
       v0 = J.getInterceptor$as(l).get$length(l);
       if (P.identical(J.getInterceptor$n(i).$lt(i, v0), true)) {
         x = J.getInterceptor$as(l).$index(l, i);
         j = 0;
         while (true) {
-          v1 = J.getInterceptor$as(x).get$length(x);
-          if (P.identical(J.getInterceptor$n(j).$lt(j, v1), true)) {
+          v0 = J.getInterceptor$as(x).get$length(x);
+          if (P.identical(J.getInterceptor$n(j).$lt(j, v0), true)) {
             P.print(J.getInterceptor$as(x).$index(x, j));
             j = J.getInterceptor$ns(j).$add(j, 1);
           } else {
             i = J.getInterceptor$ns(i).$add(i, 1);
             continue L0;
           }
         }
       } else
         return null;
     }
 }"""),
 ];
 
 
 void main() {
   runTests(tests);
 }