message: [XLA] Fix a live-range heuristic in all-gather CSE
  earlier AG's live range only needs to cover the current AG, not its last user, to be beneficial.
diff: @@ -88,7 +88,7 @@ StatusOr<bool> RunOnComputation(HloComputation* comp, bool for_replicas,
  auto& earlier_ags = operand_to_ag[ag->operand(0)];
  bool found = false;
  - int64 lowest_user_h = lowest_user_height(ag);
  + int64 ag_height = height[ag];
  for (auto& eag : earlier_ags) {
  auto old_channel_id = ag->channel_id();
  if (eag->channel_id() && ag->channel_id()) {
  @@ -100,7 +100,7 @@ StatusOr<bool> RunOnComputation(HloComputation* comp, bool for_replicas,
  }
  found = true;
  ag->set_channel_id(old_channel_id);
  - if (lowest_user_height(eag) > lowest_user_h + distance_threshold) {
  + if (lowest_user_height(eag) > ag_height + distance_threshold) {
  eag = ag;
  continue;
  }
  