dataToTag# doesn't take advantage of tag bits

Currently dataToTag# always enters its argument and looks at the info table of the result to determine the tag. Now since we tag pointers to constructors of "large" sum types (#14373 (closed)) there is potentially a significant payoff to taking better advantage of pointer tagging. That is, dataToTag# should:

if (TAG_BITS(arg) == 0)
  arg = ENTER(arg);

if (TAG_BITS(arg) == 0)
  return GET_INFO(arg).tag;
else
  return TAG_BITS(arg) - 1;

This will be particularly beneficial to user code after teaching Ord to compare enumerations via dataToTag# (#16578).