Try convert polymorphic stores into a single monomorphic store.

src/hydrogen.h

 // Copyright 2012 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 1689 matching lines @@
                                        Handle<String> name);
   HInstruction* TryLoadPolymorphicAsMonomorphic(Property* expr,
                                                 HValue* object,
                                                 SmallMapList* types,
                                                 Handle<String> name);
   void HandlePolymorphicStoreNamedField(Assignment* expr,
                                         HValue* object,
                                         HValue* value,
                                         SmallMapList* types,
                                         Handle<String> name);
+  bool TryStorePolymorphicAsMonomorphic(Assignment* expr,
+                                        HValue* object,
+                                        HValue* value,
+                                        SmallMapList* types,
+                                        Handle<String> name);
   void HandlePolymorphicCallNamed(Call* expr,
                                   HValue* receiver,
                                   SmallMapList* types,
                                   Handle<String> name);
   void HandleLiteralCompareTypeof(CompareOperation* expr,
                                   HTypeof* typeof_expr,
                                   Handle<String> check);
   void HandleLiteralCompareNil(CompareOperation* expr,
                                HValue* value,
                                NilValue nil);
@@ skipping 311 matching lines @@
   EmbeddedVector<char, 64> filename_;
   HeapStringAllocator string_allocator_;
   StringStream trace_;
   int indent_;
 };
 
 
 } }  // namespace v8::internal
 
 #endif  // V8_HYDROGEN_H_