[es6] Reintroduce the instanceof operator in the backends.

src/isolate-inl.h

Index: src/isolate-inl.h
diff --git a/src/isolate-inl.h b/src/isolate-inl.h
index 48e3f56b67e771c02d96784ccda5a392073780cd..48ea0aaeea8ac4c41a43030f295b45e2908afa48 100644
--- a/src/isolate-inl.h
+++ b/src/isolate-inl.h
@@ -120,6 +120,12 @@ bool Isolate::IsArraySpeciesLookupChainIntact() {
          Smi::cast(species_cell->value())->value() == kArrayProtectorValid;
 }
 
+bool Isolate::IsHasInstanceLookupChainIntact() {
+  if (!FLAG_harmony_instanceof) return true;
+  PropertyCell* has_instance_cell = heap()->has_instance_protector();
+  return has_instance_cell->value() == Smi::FromInt(kArrayProtectorValid);
+}
+
 }  // namespace internal
 }  // namespace v8