Introduce HTransitionElementsKind instruction.

src/objects.h

Index: src/objects.h
diff --git a/src/objects.h b/src/objects.h
index b95fa574a05c9af262f5fbd64a206a7c203d682b..b23e6c2b3c4e711ba2a815f23df20f6dc3bc44d7 100644
--- a/src/objects.h
+++ b/src/objects.h
@@ -1789,6 +1789,11 @@ class JSObject: public JSReceiver {
   MUST_USE_RESULT MaybeObject* GetElementsTransitionMap(
       ElementsKind elements_kind);
 
+  MUST_USE_RESULT MaybeObject* TransitionElementsKind(ElementsKind to_kind);
+
+  static bool IsValidElementsTransition(ElementsKind from_kind,
+                                        ElementsKind to_kind);
+
   // Converts a descriptor of any other type to a real field,
   // backed by the properties array.  Descriptors of visible
   // types, such as CONSTANT_FUNCTION, keep their enumeration order.
@@ -4288,6 +4293,11 @@ class Map: public HeapObject {
   MaybeObject* AddElementsTransition(ElementsKind elements_kind,
                                      Map* transitioned_map);
 
+  // Returns the transitioned map for this map with the most generic
+  // elements_kind that's found in |candidates|, or NULL if no match is
+  // found at all.
+  Map* FindTransitionedMap(MapList* candidates);
+
   // Dispatched behavior.
 #ifdef OBJECT_PRINT
   inline void MapPrint() {