Don't double unlink in DCE.

src/hydrogen.cc

 // Copyright 2012 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 5418 matching lines @@
   for (int i = 0; i < blocks()->length(); ++i) {
     for (HInstruction* instr = blocks()->at(i)->first();
          instr != NULL;
          instr = instr->next()) {
       if (instr->IsDead()) worklist.Add(instr, zone());
     }
   }
 
   while (!worklist.is_empty()) {
     HInstruction* instr = worklist.RemoveLast();
+    // This happens when an instruction is used multiple times as operand. That
+    // in turn could happen through GVN.
+    if (!instr->IsLinked()) continue;
     if (FLAG_trace_dead_code_elimination) {
       HeapStringAllocator allocator;
       StringStream stream(&allocator);
       instr->PrintNameTo(&stream);
       stream.Add(" = ");
       instr->PrintTo(&stream);
       PrintF("[removing dead instruction %s]\n", *stream.ToCString());
     }
     instr->DeleteAndReplaceWith(NULL);
     for (int i = 0; i < instr->OperandCount(); ++i) {
@@ skipping 6821 matching lines @@
     }
   }
 
 #ifdef DEBUG
   if (graph_ != NULL) graph_->Verify(false);  // No full verify.
   if (allocator_ != NULL) allocator_->Verify();
 #endif
 }
 
 } }  // namespace v8::internal