message: [XLA] [NFC] Clean up InitializeInstructionValueSets.
  Remove an unused parameter. Replace the use of define_top_level_only with
  define_value_at. Modify some comments for clarification.
diff: @@ -774,9 +774,9 @@ Status HloDataflowAnalysis::InitializeInstructionValueSets() {
  std::forward_as_tuple(instruction),
  std::forward_as_tuple(instruction->shape()));
  - // Lambda to set the value set to define all values in the output of the
  - // instruction.
  - auto define_all_values = [this, &instruction](bool is_phi = false) {
  + // For each sub-shape of the instruction shape, add a new HloValue to its
  + // HloValueSet.
  + auto define_all_values = [this, &instruction]() {
  for (auto& pair : GetInstructionValueSet(instruction)) {
  const ShapeIndex& index = pair.first;
  HloValue* value = NewHloValue(instruction, index, /*is_phi=*/false);
  @@ -784,16 +784,8 @@ Status HloDataflowAnalysis::InitializeInstructionValueSets() {
  }
  };
  - // Lambda to set the value set to define only the top-level buffer in the
  - // output of the instruction. Any other values flow from the operands of
  - // the instruction (or from cross-computation dataflow).
  - auto define_top_level_only = [this, &instruction]() {
  - HloValue* value =
  - NewHloValue(instruction, /*index=*/{}, /*is_phi=*/false);
  - GetValueSet(instruction, /*index=*/{}).AddValue(value);
  - };
  -
  - // Lambda to set the value set at the given index of the output.
  + // Add a new HloValue to the HloValueSet corresponding to the given index
  + // of the instruction shape.
  auto define_value_at = [this, &instruction](const ShapeIndex& index) {
  HloValue* value = NewHloValue(instruction, index, /*is_phi=*/false);
  GetValueSet(instruction, index).AddValue(value);
  @@ -840,7 +832,7 @@ Status HloDataflowAnalysis::InitializeInstructionValueSets() {
  case HloOpcode::kTuple:
  // These instructions only define their top-level values. Any other
  // values flow from their operands.
  - define_top_level_only();
  + define_value_at(/*index=*/{});
  break;
  case HloOpcode::kCopyDone:
  // CopyDone produces an element. Its output aliases its input tuple
  