message: [XLA] Check buffer definition before use
diff: @@ -1419,6 +1419,10 @@ Status LayoutAssignment::PropagateOperandConstraint(
  if (subshape.rank() != operand->shape().rank()) {
  return Status::OK();
  }
  + if (!constraints->points_to_analysis()
  + .InstructionDefinesBufferAtIndex(user, shape_index)) {
  + return Status::OK();
  + }
  // TODO(b/67641796): Are there cases except fusion that use this code
  // path?
  TF_ASSIGN_OR_RETURN(
  @@ -1450,6 +1454,10 @@ Status LayoutAssignment::PropagateOperandConstraint(
  if (subshape.rank() <= 1) {
  return Status::OK();
  }
  + if (!constraints->points_to_analysis().InstructionDefinesBufferAtIndex(
  + user, shape_index)) {
  + return Status::OK();
  + }
  TF_ASSIGN_OR_RETURN(
  const LogicalBuffer* buffer,
  constraints->points_to_analysis().GetBufferDefinedAt(user,
  