message: [XLA] Add typedef-names for the instruction sequences returned from the
  HloComputation.
  This allows the use of the alias names.
diff: @@ -198,6 +198,13 @@ class HloComputation {
  const HloComputationProto& proto,
  const absl::flat_hash_map<int64, HloComputation*>& computation_map);
  + using InstructionSequence = tensorflow::gtl::iterator_range<
  + UnwrappingIterator<std::list<std::unique_ptr<HloInstruction>>::iterator>>;
  +
  + using ConstInstructionSequence =
  + tensorflow::gtl::iterator_range<UnwrappingIterator<
  + std::list<std::unique_ptr<HloInstruction>>::const_iterator>>;
  +
  // Gets the instructions in this computation.
  //
  // The returned type is a range of HloInstruction*s, so you can iterate over
  @@ -205,15 +212,11 @@ class HloComputation {
  //
  // for (HloInstruction* instr : computation->instructions()) { ... }
  //
  - tensorflow::gtl::iterator_range<UnwrappingIterator<
  - std::list<std::unique_ptr<HloInstruction>>::const_iterator>>
  - instructions() const {
  + ConstInstructionSequence instructions() const {
  return {MakeUnwrappingIterator(instructions_.begin()),
  MakeUnwrappingIterator(instructions_.end())};
  }
  - tensorflow::gtl::iterator_range<
  - UnwrappingIterator<std::list<std::unique_ptr<HloInstruction>>::iterator>>
  - instructions() {
  + InstructionSequence instructions() {
  return {MakeUnwrappingIterator(instructions_.begin()),
  MakeUnwrappingIterator(instructions_.end())};
  }
  