[turbofan] Value numbering should never replace a node with itself.

src/compiler/value-numbering-reducer.cc

 // Copyright 2014 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 "src/compiler/value-numbering-reducer.h"
 
 #include "src/compiler/node.h"
 
 namespace v8 {
 namespace internal {
@@ skipping 42 matching lines @@
 }
 
 
 ValueNumberingReducer::~ValueNumberingReducer() {}
 
 
 Reduction ValueNumberingReducer::Reduce(Node* node) {
   Entry** head = &buckets_[HashCode(node) % arraysize(buckets_)];
   for (Entry* entry = *head; entry; entry = entry->next()) {
     if (entry->node()->op() == NULL) continue;
+    if (entry->node() == node) return NoChange();
     if (Equals(node, entry->node())) {
       return Replace(entry->node());
     }
   }
   *head = new (zone()) Entry(node, *head);
   return NoChange();
 }
 
 }  // namespace compiler
 }  // namespace internal
 }  // namespace v8