Propagate pseudo-constants expressions separately in tree rewriter.

tests/compiler/dart2js/js_backend_cps_ir_supercall_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 supercall_test;
 
 import 'js_backend_cps_ir.dart';
 
 const List<TestEntry> tests = const [
   const TestEntry.forMethod('function(Sub#m)', """
 class Base {
   m(x) {
     print(x+1);
   }
 }
 class Sub extends Base {
   m(x) => super.m(x+10);
 }
 main() {
   new Sub().m(100);
 }""",
 r"""
 function(x) {
-  var v0, v1;
-  v0 = 10;
-  v1 = J.getInterceptor$ns(x).$add(x, v0);
-  return V.Base.prototype.m$1.call(this, v1);
+  return V.Base.prototype.m$1.call(this, J.getInterceptor$ns(x).$add(x, 10));
 }"""),
 
   // Reenable when we support compiling functions that
   // need interceptor calling convention.
 // const TestEntry.forMethod('function(Sub#+)', """
 // class Base {
 //   m(x) {
 //     print(x+1000);
 //   }
 //   operator+(x) => m(x+10);
@@ skipping 30 matching lines @@
   v0 = this.field;
   return J.getInterceptor$ns(x).$add(x, v0);
 }"""),
 
 
 ];
 
 void main() {
   runTests(tests);
 }