message: [XLA] Improve runtime of MakeInstructionPostOrder by reserving the flat set
  upfront.
diff: @@ -15,8 +15,8 @@ limitations under the License.
  #include "tensorflow/compiler/xla/service/hlo_computation.h"
  -#include <stddef.h>
  #include <algorithm>
  +#include <cstddef>
  #include <functional>
  #include <list>
  #include <queue>
  @@ -396,6 +396,7 @@ std::vector<HloInstruction*> HloComputation::MakeInstructionPostOrder() const {
  post_order.reserve(instruction_count());
  std::vector<HloInstruction*> trace_instructions;
  absl::flat_hash_map<HloInstruction*, VisitState> visited;
  + visited.reserve(instruction_count());
  for (auto& instruction : instructions_) {
  if (instruction->opcode() == HloOpcode::kTrace) {
  // Trace instructions aren't handled by the DFS visitor. Add trace
  