Fix case of locality optimizer being wrong for split repeated fields and add a check that the optimized order has the same size as the unoptimized order.

PiperOrigin-RevId: 543779555

src/google/protobuf/compiler/cpp/message.cc

@@ -547,8 +547,10 @@ MessageGenerator::MessageGenerator(
     }
   }
 
+  const size_t initial_size = optimized_order_.size();
   message_layout_helper_->OptimizeLayout(&optimized_order_, options_,
                                          scc_analyzer_);
+  ABSL_CHECK_EQ(initial_size, optimized_order_.size());
 
   // This message has hasbits iff one or more fields need one.
   for (auto field : optimized_order_) {