Fix Node::ReplaceUses to handle unused nodes.

src/compiler/generic-node-inl.h

 // 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.
 
 #ifndef V8_COMPILER_GENERIC_NODE_INL_H_
 #define V8_COMPILER_GENERIC_NODE_INL_H_
 
 #include "src/v8.h"
 
 #include "src/compiler/generic-graph.h"
@@ skipping 46 matching lines @@
 }
 
 template <class B, class S>
 void GenericNode<B, S>::ReplaceUses(GenericNode* replace_to) {
   for (Use* use = first_use_; use != NULL; use = use->next) {
     use->from->GetInputRecordPtr(use->input_index)->to = replace_to;
   }
   if (replace_to->last_use_ == NULL) {
     DCHECK_EQ(NULL, replace_to->first_use_);
     replace_to->first_use_ = first_use_;
-  } else {
+    replace_to->last_use_ = last_use_;
+  } else if (first_use_ != NULL) {
     DCHECK_NE(NULL, replace_to->first_use_);
     replace_to->last_use_->next = first_use_;
     first_use_->prev = replace_to->last_use_;
+    replace_to->last_use_ = last_use_;
   }
-  replace_to->last_use_ = last_use_;
   replace_to->use_count_ += use_count_;
   use_count_ = 0;
   first_use_ = NULL;
   last_use_ = NULL;
 }
 
 template <class B, class S>
 template <class UnaryPredicate>
 void GenericNode<B, S>::ReplaceUsesIf(UnaryPredicate pred,
                                       GenericNode* replace_to) {
@@ skipping 153 matching lines @@
     ++use;
     ++input;
   }
   return result;
 }
 }
 }
 }  // namespace v8::internal::compiler
 
 #endif  // V8_COMPILER_GENERIC_NODE_INL_H_