Reuse backing store of two zone lists by using Rewind(0) instead of Clear().

src/hydrogen.cc

 // Copyright 2011 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 622 matching lines @@
 
 
 void HGraph::OrderBlocks() {
   HPhase phase("Block ordering");
   BitVector visited(blocks_.length());
 
   ZoneList<HBasicBlock*> reverse_result(8);
   HBasicBlock* start = blocks_[0];
   Postorder(start, &visited, &reverse_result, NULL);
 
-  blocks_.Clear();
+  blocks_.Rewind(0);
   int index = 0;
   for (int i = reverse_result.length() - 1; i >= 0; --i) {
     HBasicBlock* b = reverse_result[i];
     blocks_.Add(b);
     b->set_block_id(index++);
   }
 }
 
 
 void HGraph::PostorderLoopBlocks(HLoopInformation* loop,
@@ skipping 5236 matching lines @@
     }
   }
 
 #ifdef DEBUG
   if (graph_ != NULL) graph_->Verify();
   if (allocator_ != NULL) allocator_->Verify();
 #endif
 }
 
 } }  // namespace v8::internal