[turbofan] Restrict the broker's kRetired mode.

Once the broker retired, only allow retrieval of a reference's
handle. That's all we need for now.

Bug: v8:7790
Change-Id: Ib75887ed4a68e19ad7fad4c6046e340502542850
Reviewed-on: https://chromium-review.googlesource.com/1251086Reviewed-by: Maya Lekova <mslekova@chromium.org>
Reviewed-by: Jaroslav Sevcik <jarin@chromium.org>
Commit-Queue: Georg Neis <neis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#56308}

src/compiler/js-heap-broker.cc

@@ -1379,6 +1379,7 @@ void JSHeapBroker::SetNativeContextRef() {
 
 void JSHeapBroker::SerializeStandardObjects() {
   if (mode() == kDisabled) return;
+  CHECK_EQ(mode(), kSerializing);
 
   TraceScope tracer(this, "JSHeapBroker::SerializeStandardObjects");
 
@@ -1574,7 +1575,7 @@ base::Optional<MapRef> MapRef::AsElementsKind(ElementsKind kind) const {
     AllowHandleDereference allow_handle_dereference;
     return MapRef(broker(), Map::AsElementsKind(broker()->isolate(),
                                                 object<Map>(), kind));
-  } else {
+  }
   if (kind == elements_kind()) return *this;
   const ZoneVector<MapData*>& elements_kind_generalizations =
       data()->AsMap()->elements_kind_generalizations();
@@ -1583,7 +1584,6 @@ base::Optional<MapRef> MapRef::AsElementsKind(ElementsKind kind) const {
     if (map.elements_kind() == kind) return map;
   }
   return base::Optional<MapRef>();
-  }
 }
 
 int JSFunctionRef::InitialMapInstanceSizeWithMinSlack() const {
@@ -2207,7 +2207,7 @@ ObjectRef JSRegExpRef::source() const {
   return ObjectRef(broker(), ObjectRef::data()->AsJSRegExp()->source());
 }
 
-Handle<Object> ObjectRef::object() const { return data()->object(); }
+Handle<Object> ObjectRef::object() const { return data_->object(); }
 
 JSHeapBroker* ObjectRef::broker() const { return broker_; }
 
@@ -2221,7 +2221,7 @@ ObjectData* ObjectRef::data() const {
       CHECK_NE(data_->kind(), kUnserializedHeapObject);
       return data_;
     case JSHeapBroker::kRetired:
-      return data_;
+      UNREACHABLE();
   }
 }