Introduce FeedbackVectorSlot type - better than int.

src/compiler/js-operator.h

Index: src/compiler/js-operator.h
diff --git a/src/compiler/js-operator.h b/src/compiler/js-operator.h
index 3f94ff679b71b909cbce0fb3e75685e2c5ee2193..8cd9fc7605f8b9327263a98304d5ae36e9cc2c09 100644
--- a/src/compiler/js-operator.h
+++ b/src/compiler/js-operator.h
@@ -98,15 +98,15 @@ ContextAccess const& ContextAccessOf(Operator const*);
 
 class VectorSlotPair {
  public:
-  VectorSlotPair(Handle<TypeFeedbackVector> vector, int slot)
+  VectorSlotPair(Handle<TypeFeedbackVector> vector, FeedbackVectorSlot slot)
       : vector_(vector), slot_(slot) {}
 
   Handle<TypeFeedbackVector> vector() const { return vector_; }
-  int slot() const { return slot_; }
+  FeedbackVectorSlot slot() const { return slot_; }
 
  private:
   const Handle<TypeFeedbackVector> vector_;
-  const int slot_;
+  const FeedbackVectorSlot slot_;
 };