message: [XLA] [NFC] Explain decisions not to remove copies in logging
diff: @@ -703,6 +703,9 @@ class CopyRemover {
  if (next_dest != nullptr) {
  // Live range of 'from' value (s_x) must be before 'next_dest' (d_1);
  if (!LiveRangeBefore(*src, *next_dest)) {
  + VLOG(2) << "Not removing the copy: live range of "
  + << src->value->ToShortString() << " is not before "
  + << next_dest->value->ToShortString();
  return false;
  }
  }
  @@ -713,6 +716,9 @@ class CopyRemover {
  ValueNode* last_dest = dest->prev;
  DCHECK(IsTail(*last_dest));
  if (!LiveRangeBefore(*last_dest, *next_src)) {
  + VLOG(2) << "Not removing the copy: live range of "
  + << last_dest->value->ToShortString() << " is not before "
  + << next_src->value->ToShortString();
  return false;
  }
  }
  @@ -743,12 +749,18 @@ class CopyRemover {
  DCHECK(IsHead(*first_src));
  if (!LiveRangeBefore(*prev_dest, *first_src)) {
  // Live range of value d_{y-1} is not before s_0.
  + VLOG(2) << "Not removing the copy: live range of "
  + << prev_dest->value->ToShortString() << " is not before "
  + << first_src->value->ToShortString();
  return false;
  }
  ValueNode* next_dest = Next(*dest);
  if (next_dest != nullptr) {
  if (!LiveRangeBefore(*src, *next_dest)) {
  // Live range of value s_n is not before d_{y+1}.
  + VLOG(2) << "Not removing the copy: live range of "
  + << src->value->ToShortString() << " is not before "
  + << next_dest->value->ToShortString();
  return false;
  }
  }
  