Propagate pseudo-constants expressions separately in tree rewriter.

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';
 
 const List<TestEntry> tests = const [
   const TestEntry("""
 main() {
   var g = 1;
 
   var x = g + 3;
   print(x);
 }""",
 r"""
 function() {
-  var g, v0;
+  var g;
   g = 1;
-  v0 = 3;
-  P.print(J.getInterceptor$ns(g).$add(g, v0));
+  P.print(J.getInterceptor$ns(g).$add(g, 3));
   return null;
 }"""),
   const TestEntry("""
 main() {
   var l = ['hest', ['h', 'e', 's', 't']];
   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, v2, v3;
+  var l, i, v0, x, j, v1;
   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)) {
             P.print(J.getInterceptor$as(x).$index(x, j));
-            v2 = 1;
-            j = J.getInterceptor$ns(j).$add(j, v2);
+            j = J.getInterceptor$ns(j).$add(j, 1);
           } else {
-            v3 = 1;
-            i = J.getInterceptor$ns(i).$add(i, v3);
+            i = J.getInterceptor$ns(i).$add(i, 1);
             continue L0;
           }
         }
       } else
         return null;
     }
 }"""),
 ];
 
 
 void main() {
   runTests(tests);
 }