dart2js cps: Remove redundant traversal from variable merger pass.

pkg/compiler/lib/src/tree_ir/optimization/variable_merger.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.
 
 library tree_ir.optimization.variable_merger;
 
 import 'optimization.dart' show Pass;
 import '../tree_ir_nodes.dart';
 
 /// Merges variables based on liveness and source variable information.
 ///
 /// This phase cleans up artifacts introduced by the translation through CPS,
 /// where each source variable is translated into several copies. The copies
 /// are merged again when they are not live simultaneously.
-class VariableMerger extends RecursiveVisitor implements Pass {
+class VariableMerger implements Pass {
   String get passName => 'Variable merger';
 
   void rewrite(FunctionDefinition node) {
-    rewriteFunction(node);
-    visitStatement(node.body);
-  }
-
-  /// Rewrites the given function.
-  /// This is called for the outermost function and inner functions.
-  void rewriteFunction(FunctionDefinition node) {
     BlockGraphBuilder builder = new BlockGraphBuilder();
     builder.build(node);
     _computeLiveness(builder.blocks);
     Map<Variable, Variable> subst =
         _computeRegisterAllocation(builder.blocks, node.parameters);
     new SubstituteVariables(subst).apply(node);
   }
 }
 
 /// A read or write access to a variable.
@@ skipping 485 matching lines @@
     node.expression = visitExpression(node.expression);
     node.next = visitStatement(node.next);
     if (node.expression is VariableUse) {
       VariableUse use = node.expression;
       --use.variable.readCount;
       return node.next;
     }
     return node;
   }
 }