Make use lists into doubly-linked lists.

runtime/vm/intermediate_language.h

Index: runtime/vm/intermediate_language.h
===================================================================
--- runtime/vm/intermediate_language.h	(revision 17923)
+++ runtime/vm/intermediate_language.h	(working copy)
@@ -73,6 +73,7 @@
  public:
   explicit Value(Definition* definition)
       : definition_(definition),
+        previous_use_(NULL),
         next_use_(NULL),
         instruction_(NULL),
         use_index_(-1),
@@ -81,6 +82,9 @@
   Definition* definition() const { return definition_; }
   void set_definition(Definition* definition) { definition_ = definition; }
 
+  Value* previous_use() const { return previous_use_; }
+  void set_previous_use(Value* previous) { previous_use_ = previous; }
+
   Value* next_use() const { return next_use_; }
   void set_next_use(Value* next) { next_use_ = next; }
 
@@ -133,6 +137,7 @@
 
  private:
   Definition* definition_;
+  Value* previous_use_;
   Value* next_use_;
   Instruction* instruction_;
   intptr_t use_index_;