Relax TransitionElementsKind DependsOn/Changes dependencies.

src/hydrogen.cc

Index: src/hydrogen.cc
diff --git a/src/hydrogen.cc b/src/hydrogen.cc
index a6b60b1f77a8fc439409096fbd88cd6c7b79006c..0628bd7638332df3f8028bfc5b4e8ae056dc1c49 100644
--- a/src/hydrogen.cc
+++ b/src/hydrogen.cc
@@ -1192,7 +1192,15 @@ void HValueMap::Kill(GVNFlagSet flags) {
 
       // Now possibly drop directly indexed element.
       value = array_[i].value;
-      if (value->gvn_flags().ContainsAnyOf(depends_flags)) {  // Drop it.
+      // Remove a value from the value map is one of its dependencies has been
+      // changed, unless it's a TransitionElementsKind, in which case it's safe
+      // to keep it. TransitionElementsKinds that are dominated by another
+      // equivalent transition, even if there is an intervening side-effect.
+      // TransitionElementsKind only has a one-time side effect, so all
+      // subsequent Transitions are no-ops, even if there are other side effects
+      // in between.
+      if (value->gvn_flags().ContainsAnyOf(depends_flags) &&
+          !value->IsTransitionElementsKind()) {  // Drop it.
         count_--;
         int head = array_[i].next;
         if (head == kNil) {
@@ -4454,7 +4462,7 @@ HValue* HGraphBuilder::HandlePolymorphicElementAccess(HValue* object,
     Handle<Map> map = maps->at(i);
     ASSERT(map->IsMap());
     if (!transition_target.at(i).is_null()) {
-      object = AddInstruction(new(zone()) HTransitionElementsKind(
+      AddInstruction(new(zone()) HTransitionElementsKind(
           object, map, transition_target.at(i)));
     } else {
       type_todo[map->elements_kind()] = true;