Fix Node::ReplaceUses to handle unused nodes.

test/cctest/compiler/test-node.cc

 // Copyright 2013 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include <functional>
 
 #include "src/v8.h"
 
 #include "graph-tester.h"
 #include "src/compiler/generic-node-inl.h"
@@ skipping 100 matching lines @@
   Node::Uses::iterator i(n0->uses().begin());
   CHECK_EQ(n1, *i);
   ++i;
   CHECK(n0->uses().end() == i);
 }
 
 
 TEST(NodeUseIteratorReplaceNoUses) {
   GraphTester graph;
   Node* n0 = graph.NewNode(&dummy_operator);
-  Node* n3 = graph.NewNode(&dummy_operator);
-  n0->ReplaceUses(n3);
+  Node* n1 = graph.NewNode(&dummy_operator);
+  Node* n2 = graph.NewNode(&dummy_operator);
+  Node* n3 = graph.NewNode(&dummy_operator, n2);
+  n0->ReplaceUses(n1);
   CHECK(n0->uses().begin() == n0->uses().end());
+  n0->ReplaceUses(n2);
+  CHECK(n0->uses().begin() == n0->uses().end());
+  USE(n3);
 }
 
 
 TEST(NodeUseIteratorReplaceUses) {
   GraphTester graph;
   Node* n0 = graph.NewNode(&dummy_operator);
   Node* n1 = graph.NewNode(&dummy_operator, n0);
   Node* n2 = graph.NewNode(&dummy_operator, n0);
   Node* n3 = graph.NewNode(&dummy_operator);
   Node::Uses::iterator i1(n0->uses().begin());
@@ skipping 672 matching lines @@
     n1->ReplaceInput(0, n1);  // self-reference.
 
     CHECK_EQ(0, n0->UseCount());
     CHECK_EQ(1, n1->UseCount());
     n1->RemoveAllInputs();
     CHECK_EQ(1, n1->InputCount());
     CHECK_EQ(0, n1->UseCount());
     CHECK_EQ(NULL, n1->InputAt(0));
   }
 }