dart2js cps: Retain refinement nodes and update refinements after GVN.

pkg/compiler/lib/src/cps_ir/remove_refinements.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 cps_ir.optimization.remove_refinements;
-
-import 'optimizers.dart' show Pass;
-import 'cps_ir_nodes.dart';
-
-/// Removes all [Refinement] nodes from the IR.
-///
-/// This simplifies subsequent passes that don't rely on path-sensitive
-/// type information but depend on equality between primitives.
-class RemoveRefinements extends TrampolineRecursiveVisitor implements Pass {
-  String get passName => 'Remove refinement nodes';
-
-  void rewrite(FunctionDefinition node) {
-    visit(node);
-  }
-
-  @override
-  Expression traverseLetPrim(LetPrim node) {
-    Expression next = node.body;
-    if (node.primitive is Refinement) {
-      Refinement refinement = node.primitive;
-      Primitive value = refinement.value.definition;
-      if (refinement.hint != null && value.hint == null) {
-        value.hint = refinement.hint;
-      }
-      refinement..replaceUsesWith(value)..destroy();
-      node.remove();
-    }
-    return next;
-  }
-}