[turbofan] Add support for extending properties backing store.

src/compiler/access-info.cc

Index: src/compiler/access-info.cc
diff --git a/src/compiler/access-info.cc b/src/compiler/access-info.cc
index ceeec5e3cb5a1cd3d6d008ea5a46ce27ae7ceacd..bbaa667147ce91afe793f2d6ba8407ae64bf4b2e 100644
--- a/src/compiler/access-info.cc
+++ b/src/compiler/access-info.cc
@@ -515,10 +515,6 @@ 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)) {