message: [XLA] Make a LOG(FATAL) a bit more useful.
diff: @@ -1403,7 +1403,7 @@ int64 HloInstruction::operand_index(const HloInstruction* target) const {
  return i;
  }
  }
  - LOG(FATAL) << "target was not an operand";
  + LOG(FATAL) << "target was not an operand: " << target->ToString();
  }
  Status HloInstruction::AddControlDependencyTo(HloInstruction* instruction) {
  