Use pointer tag in dataToTag#

While looking at !2873 I noticed that dataToTag# previously didn't look
at a pointer's tag to determine its constructor. To be fair, there is a
bit of a trade-off here: using the pointer tag requires a bit more code
and another branch. On the other hand, it allows us to eliminate looking
at the info table in many cases (especially now since we tag large
constructor families; see #14373).