Add a backward instruction iterator and use it in the liveness analysis.

vm/bit_vector.h

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 #ifndef VM_BIT_VECTOR_H_
 #define VM_BIT_VECTOR_H_
 
 #include "vm/allocation.h"
 #include "vm/isolate.h"
 #include "vm/zone.h"
@@ skipping 45 matching lines @@
 
   static intptr_t SizeFor(intptr_t length) {
     return 1 + ((length - 1) / kBitsPerWord);
   }
 
   void Add(intptr_t i) {
     ASSERT(i >= 0 && i < length());
     data_[i / kBitsPerWord] |= (static_cast<uword>(1) << (i % kBitsPerWord));
   }
 
+  void Remove(intptr_t i) {
+    ASSERT(i >= 0 && i < length());
+    data_[i / kBitsPerWord] &= ~(static_cast<uword>(1) << (i % kBitsPerWord));
+  }
+
   // Add all elements that are in the bitvector from.
   bool AddAll(BitVector* from);
 
   // From the bitvector gen add those elements that are not in the
   // bitvector kill.
   bool KillAndAdd(BitVector* kill, BitVector* gen);
 
   bool Contains(int i) const {
     ASSERT(i >= 0 && i < length());
     uword block = data_[i / kBitsPerWord];
@@ skipping 12 matching lines @@
   intptr_t length_;
   intptr_t data_length_;
   uword* data_;
 
   DISALLOW_COPY_AND_ASSIGN(BitVector);
 };
 
 }  // namespace dart
 
 #endif  // VM_BIT_VECTOR_H_