[turbofan] Don't need any write barrier when storing maps from the root set.

Maps that are immortal immovable (i.e. the one pointer filler map) don't
need write barriers.

R=jarin@chromium.org

Review-Url: https://codereview.chromium.org/1945023003
Cr-Commit-Position: refs/heads/master@{#36016}

src/compiler/simplified-lowering.cc

@@ -733,10 +733,6 @@ class RepresentationSelector {
           value_type->AsConstant()->Value()->IsHeapObject()) {
         Handle<HeapObject> value_object =
             Handle<HeapObject>::cast(value_type->AsConstant()->Value());
-        if (value_object->IsMap()) {
-          // Write barriers for storing maps are cheaper.
-          return kMapWriteBarrier;
-        }
         RootIndexMap root_index_map(jsgraph_->isolate());
         int root_index = root_index_map.Lookup(*value_object);
         if (root_index != RootIndexMap::kInvalidRootIndex &&
@@ -744,6 +740,10 @@ class RepresentationSelector {
           // Write barriers are unnecessary for immortal immovable roots.
           return kNoWriteBarrier;
         }
+        if (value_object->IsMap()) {
+          // Write barriers for storing maps are cheaper.
+          return kMapWriteBarrier;
+        }
       }
       if (field_type->Is(Type::TaggedPointer()) ||
           value_type->Is(Type::TaggedPointer())) {