New hydrogen instruction to reduce cost of growing an array on keyed stores.

src/interface-descriptors.h

Index: src/interface-descriptors.h
diff --git a/src/interface-descriptors.h b/src/interface-descriptors.h
index ab1517e0df6400bd7805bb2c31f8f63be3758e7a..418eb0c488fddab21bf517f8b8b89f6256094674 100644
--- a/src/interface-descriptors.h
+++ b/src/interface-descriptors.h
@@ -538,10 +538,9 @@ class GrowArrayElementsDescriptor : public CallInterfaceDescriptor {
  public:
   DECLARE_DESCRIPTOR(GrowArrayElementsDescriptor, CallInterfaceDescriptor)
 
-  enum RegisterInfo { kObjectIndex, kKeyIndex, kCapacityIndex };
+  enum RegisterInfo { kObjectIndex, kKeyIndex };
   static const Register ObjectRegister();
   static const Register KeyRegister();
-  static const Register CapacityRegister();
 };
 
 #undef DECLARE_DESCRIPTOR