Don't double unlink in DCE.

src/hydrogen.cc

Index: src/hydrogen.cc
diff --git a/src/hydrogen.cc b/src/hydrogen.cc
index 1b02a62c4370147bf4a7dd83f397391bdf757ad7..58a9b78750bb7401bfdc9e167800f8f01d3ef078 100644
--- a/src/hydrogen.cc
+++ b/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);