message: [XLA] Don't allocate vector in loop
  Encourage reusing the vector's allocation rather than allocating and freeing lots of vectors with different sizes.
diff: @@ -90,8 +90,8 @@ std::unique_ptr<HloReachabilityMap> HloReachabilityMap::Build(
  auto result = absl::make_unique<HloReachabilityMap>(all);
  auto channel_group = computation->ComputeChannelDependencies();
  - for (const HloInstruction* hlo : all) {
  std::vector<HloInstruction*> inputs;
  +
  const auto add_input = [&channel_group, &inputs](HloInstruction* input) {
  inputs.push_back(input);
  if (input->opcode() == HloOpcode::kAllReduce && input->channel_id()) {
  @@ -111,6 +111,8 @@ std::unique_ptr<HloReachabilityMap> HloReachabilityMap::Build(
  }
  };
  + for (const HloInstruction* hlo : all) {
  + inputs.clear();
  add_dependencies(hlo);
  switch (hlo->opcode()) {
  