Added a simple dead code removal phase.

src/hydrogen.cc

Index: src/hydrogen.cc
diff --git a/src/hydrogen.cc b/src/hydrogen.cc
index 1afa6603aaa31a308969e9f7a15ba92e67e7d61f..628d987a3408b28bca85d250c57d8dfaf710692e 100644
--- a/src/hydrogen.cc
+++ b/src/hydrogen.cc
@@ -3358,6 +3358,7 @@ bool HGraph::Optimize(SmartArrayPointer<char>* bailout_reason) {
 
   EliminateRedundantBoundsChecks();
   DehoistSimpleArrayIndexComputations();
+  if (FLAG_dead_code_elimination) DeadCodeElimination();
 
   return true;
 }
@@ -3787,6 +3788,36 @@ void HGraph::DehoistSimpleArrayIndexComputations() {
 }
 
 
+void HGraph::DeadCodeElimination() {
+  HPhase phase("H_Dead code elimination", this);
+  ZoneList<HInstruction*> worklist(blocks_.length(), zone());
+  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();
+    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) {
+      HValue* operand = instr->OperandAt(i);
+      if (operand->IsDead()) worklist.Add(HInstruction::cast(operand), zone());
+    }
+  }
+}
+
+
 HInstruction* HGraphBuilder::AddInstruction(HInstruction* instr) {
   ASSERT(current_block() != NULL);
   current_block()->AddInstruction(instr);