[turbofan] Use unreliable receiver maps for __proto__ lowering.

src/compiler/js-call-reducer.cc

Index: src/compiler/js-call-reducer.cc
diff --git a/src/compiler/js-call-reducer.cc b/src/compiler/js-call-reducer.cc
index bfba7c9761bc0e4c9c6974903e9c7c9eaf0b9739..f0febc4d26ac61f2fecc05a409b3f7c7b158e855 100644
--- a/src/compiler/js-call-reducer.cc
+++ b/src/compiler/js-call-reducer.cc
@@ -337,7 +337,7 @@ Reduction JSCallReducer::ReduceObjectPrototypeGetProto(Node* node) {
   ZoneHandleSet<Map> receiver_maps;
   NodeProperties::InferReceiverMapsResult result =
       NodeProperties::InferReceiverMaps(receiver, effect, &receiver_maps);
-  if (result == NodeProperties::kReliableReceiverMaps) {
+  if (result != NodeProperties::kNoReceiverMaps) {
     Handle<Map> candidate_map(
         receiver_maps[0]->GetPrototypeChainRootMap(isolate()));
     Handle<Object> candidate_prototype(candidate_map->prototype(), isolate());
@@ -352,6 +352,15 @@ Reduction JSCallReducer::ReduceObjectPrototypeGetProto(Node* node) {
           receiver_map->prototype() != *candidate_prototype) {
         return NoChange();
       }
+      if (result == NodeProperties::kUnreliableReceiverMaps &&
+          !receiver_map->is_stable()) {
+        return NoChange();
+      }
+    }
+    if (result == NodeProperties::kUnreliableReceiverMaps) {
+      for (size_t i = 0; i < receiver_maps.size(); ++i) {
+        dependencies()->AssumeMapStable(receiver_maps[i]);
+      }
     }
     Node* value = jsgraph()->Constant(candidate_prototype);
     ReplaceWithValue(node, value);