Make UsePosition::hint immutable.

src/lithium-allocator.h

Index: src/lithium-allocator.h
diff --git a/src/lithium-allocator.h b/src/lithium-allocator.h
index 8b45531d9b2ed9afc5024afe77c530fd319edabb..73e09f18e2ff5739d77a25e80333128ed09410ee 100644
--- a/src/lithium-allocator.h
+++ b/src/lithium-allocator.h
@@ -244,13 +244,12 @@ class UseInterval: public ZoneObject {
 // Representation of a use position.
 class UsePosition: public ZoneObject {
  public:
-  UsePosition(LifetimePosition pos, LOperand* operand);
+  UsePosition(LifetimePosition pos, LOperand* operand, LOperand* hint);
 
   LOperand* operand() const { return operand_; }
   bool HasOperand() const { return operand_ != NULL; }
 
   LOperand* hint() const { return hint_; }
-  void set_hint(LOperand* hint) { hint_ = hint; }
   bool HasHint() const;
   bool RequiresRegister() const;
   bool RegisterIsBeneficial() const;
@@ -367,9 +366,10 @@ class LiveRange: public ZoneObject {
   void AddUseInterval(LifetimePosition start,
                       LifetimePosition end,
                       Zone* zone);
-  UsePosition* AddUsePosition(LifetimePosition pos,
-                              LOperand* operand,
-                              Zone* zone);
+  void AddUsePosition(LifetimePosition pos,
+                      LOperand* operand,
+                      LOperand* hint,
+                      Zone* zone);
 
   // Shorten the most recently added interval by setting a new start.
   void ShortenTo(LifetimePosition start);