message: Change user_set to an absl::flat_hash_set in HloInstruction.
  absl::flat_hash_set have better performance than a std::unordered_set, which can improve overall compile time.
diff: @@ -28,11 +28,10 @@ limitations under the License.
  #include <set>
  #include <string>
  #include <tuple>
  -#include <unordered_map>
  -#include <unordered_set>
  #include <vector>
  #include "absl/container/flat_hash_map.h"
  +#include "absl/container/flat_hash_set.h"
  #include "absl/container/inlined_vector.h"
  #include "absl/memory/memory.h"
  #include "absl/strings/str_cat.h"
  @@ -1645,7 +1644,7 @@ class HloInstruction {
  // members. The set enables fast membership testing and the vector enables
  // fast, stable iteration.
  std::vector<HloInstruction*> users_;
  - std::unordered_set<const HloInstruction*> user_set_;
  + absl::flat_hash_set<const HloInstruction*> user_set_;
  // The set of control successors of this instruction.
  std::vector<HloInstruction*> control_successors_;
  