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

vm/intermediate_language.h

Index: vm/intermediate_language.h
===================================================================
--- vm/intermediate_language.h	(revision 9874)
+++ vm/intermediate_language.h	(working copy)
@@ -1950,6 +1950,7 @@
  public:
   explicit ForwardInstructionIterator(BlockEntryInstr* block_entry)
       : block_entry_(block_entry), current_(block_entry) {
+    ASSERT(block_entry_->last_instruction()->next() == NULL);
     Advance();
   }
 
@@ -1958,9 +1959,7 @@
     current_ = current_->next();
   }
 
-  bool Done() const {
-    return current_ == block_entry_->last_instruction()->next();
-  }
+  bool Done() const { return current_ == NULL; }
 
   // Removes 'current_' from graph and sets 'current_' to previous instruction.
   void RemoveCurrentFromGraph();
@@ -1973,6 +1972,28 @@
 };
 
 
+class BackwardInstructionIterator : public ValueObject {
+ public:
+  explicit BackwardInstructionIterator(BlockEntryInstr* block_entry)
+      : block_entry_(block_entry), current_(block_entry->last_instruction()) {
+    ASSERT(block_entry_->previous() == NULL);
+  }
+
+  void Advance() {
+    ASSERT(!Done());
+    current_ = current_->previous();
+  }
+
+  bool Done() const { return current_ == block_entry_; }
+
+  Instruction* Current() const { return current_; }
+
+ private:
+  BlockEntryInstr* block_entry_;
+  Instruction* current_;
+};
+
+
 class GraphEntryInstr : public BlockEntryInstr {
  public:
   explicit GraphEntryInstr(TargetEntryInstr* normal_entry)