[turbofan] Eliminate redundant elements kind transitions.

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/js-graph.h"
 #include "src/compiler/node-properties.h"
 #include "src/compiler/simplified-operator.h"
 
@@ skipping 375 matching lines @@
 }
 
 Reduction LoadElimination::ReduceTransitionElementsKind(Node* node) {
   Node* const object = NodeProperties::GetValueInput(node, 0);
   Node* const source_map = NodeProperties::GetValueInput(node, 1);
   Node* const target_map = NodeProperties::GetValueInput(node, 2);
   Node* const effect = NodeProperties::GetEffectInput(node);
   AbstractState const* state = node_states_.Get(effect);
   if (state == nullptr) return NoChange();
   if (Node* const object_map = state->LookupField(object, 0)) {
+    if (target_map == object_map) {
+      // The {object} already has the {target_map}, so this TransitionElements
+      // {node} is fully redundant (independent of what {source_map} is).
+      return Replace(effect);
+    }
     state = state->KillField(object, 0, zone());
     if (source_map == object_map) {
       state = state->AddField(object, 0, target_map, zone());
     }
   } else {
     state = state->KillField(object, 0, zone());
   }
   ElementsTransition transition = ElementsTransitionOf(node->op());
   switch (transition) {
     case ElementsTransition::kFastTransition:
@@ skipping 285 matching lines @@
   DCHECK_EQ(kTaggedBase, access.base_is_tagged);
   DCHECK_EQ(0, access.offset % kPointerSize);
   int field_index = access.offset / kPointerSize;
   if (field_index >= static_cast<int>(kMaxTrackedFields)) return -1;
   return field_index;
 }
 
 }  // namespace compiler
 }  // namespace internal
 }  // namespace v8