[turbofan] Do not eagerly kill elements for slow transitions in loop state.

src/compiler/load-elimination.cc

 // Copyright 2016 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "src/compiler/load-elimination.h"
 
 #include "src/compiler/common-operator.h"
 #include "src/compiler/js-graph.h"
 #include "src/compiler/node-properties.h"
 #include "src/compiler/simplified-operator.h"
@@ skipping 989 matching lines @@
             break;
           }
           case IrOpcode::kTransitionElementsKind: {
             ElementsTransition transition = ElementsTransitionOf(current->op());
             Node* const object = NodeProperties::GetValueInput(current, 0);
             ZoneHandleSet<Map> object_maps;
             if (!state->LookupMaps(object, &object_maps) ||
                 !ZoneHandleSet<Map>(transition.target())
                      .contains(object_maps)) {
               state = state->KillMaps(object, zone());
-              state = state->KillField(
-                  object, FieldIndexOf(JSObject::kElementsOffset), zone());
+              switch (transition.mode()) {
+                case ElementsTransition::kFastTransition:
+                  break;
+                case ElementsTransition::kSlowTransition:
+                  // Kill the elements as well.
+                  state = state->KillField(
+                      object, FieldIndexOf(JSObject::kElementsOffset), zone());
+                  break;
+              }
             }
             break;
           }
           case IrOpcode::kStoreField: {
             FieldAccess const& access = FieldAccessOf(current->op());
             Node* const object = NodeProperties::GetValueInput(current, 0);
             if (access.offset == HeapObject::kMapOffset) {
               // Invalidate what we know about the {object}s map.
               state = state->KillMaps(object, zone());
             } else {
@@ skipping 82 matching lines @@
   return jsgraph()->common();
 }
 
 Graph* LoadElimination::graph() const { return jsgraph()->graph(); }
 
 Factory* LoadElimination::factory() const { return jsgraph()->factory(); }
 
 }  // namespace compiler
 }  // namespace internal
 }  // namespace v8