message: Improve the performace of HloReachabilityMap::SetReachabilityToUnionHelper
  After this change we avoid doing the expensive elementwise OR-ing if the
  input and the instruction are the same.
diff: @@ -54,9 +54,11 @@ void HloReachabilityMap::SetReachabilityToUnionHelper(
  }
  bit_vector->Set(GetIndex(instruction));
  for (const HloInstruction* input : inputs) {
  + if (input != instruction) {
  bit_vector->OrWith(GetBitVector(input));
  }
  }
  +}
  void HloReachabilityMap::SetReachable(const HloInstruction* a,
  const HloInstruction* b) {
  