[turbofan] Add support for extending properties backing store.

src/compiler/access-info.cc

 // Copyright 2015 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include <ostream>
 
 #include "src/accessors.h"
 #include "src/compilation-dependencies.h"
 #include "src/compiler/access-info.h"
 #include "src/compiler/type-cache.h"
@@ skipping 497 matching lines @@
     return true;
   }
   return false;
 }
 
 
 bool AccessInfoFactory::LookupTransition(Handle<Map> map, Handle<Name> name,
                                          MaybeHandle<JSObject> holder,
                                          PropertyAccessInfo* access_info) {
   // Check if the {map} has a data transition with the given {name}.
-  if (map->unused_property_fields() == 0) {
-    *access_info = PropertyAccessInfo::Generic(MapList{map});
-    return true;
-  }
   Handle<Map> transition_map;
   if (TransitionArray::SearchTransition(map, kData, name, NONE)
           .ToHandle(&transition_map)) {
     int const number = transition_map->LastAdded();
     PropertyDetails const details =
         transition_map->instance_descriptors()->GetDetails(number);
     // Don't bother optimizing stores to read-only properties.
     if (details.IsReadOnly()) return false;
     // TODO(bmeurer): Handle transition to data constant?
     if (details.location() != kField) return false;
@@ skipping 40 matching lines @@
   }
   return false;
 }
 
 
 Factory* AccessInfoFactory::factory() const { return isolate()->factory(); }
 
 }  // namespace compiler
 }  // namespace internal
 }  // namespace v8