[turbofan] Improve handling of TransitionElementsKind in load elimination.

src/compiler/load-elimination.cc

Index: src/compiler/load-elimination.cc
diff --git a/src/compiler/load-elimination.cc b/src/compiler/load-elimination.cc
index e50ebe19190c0313cd5c38f190199bf675655b76..0110b52f25b3f3cda7d032805d179011d6109602 100644
--- a/src/compiler/load-elimination.cc
+++ b/src/compiler/load-elimination.cc
@@ -866,6 +866,12 @@ LoadElimination::AbstractState const* LoadElimination::ComputeLoopState(
           }
           case IrOpcode::kTransitionElementsKind: {
             Node* const object = NodeProperties::GetValueInput(current, 0);
+            Node* const target_map = NodeProperties::GetValueInput(current, 2);
+            Node* const object_map = state->LookupField(
+                object, FieldIndexOf(HeapObject::kMapOffset));
+            if (target_map == object_map) {

[Benedikt Meurer, 2016/11/16 20:22:19]

Nit: Please do 

if (target_map != object_map) {
  ... kill
}
break;

instead

+              break;
+            }
             state = state->KillField(
                 object, FieldIndexOf(HeapObject::kMapOffset), zone());
             state = state->KillField(