Don't double unlink in DCE.

Review URL: https://chromiumcodereview.appspot.com/14914004

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@14545 ce2b1a6d-e550-0410-aec6-3dcde31c8c00

src/hydrogen.cc

@@ -5436,6 +5436,9 @@ void HGraph::DeadCodeElimination() {
 
   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);