isDataTyCon advertises that it's true of "data types that are
definitely represented by heap-allocated constructors.  These are
srcutinised by Core-level @case@ expressions, and they get info tables
allocated for them."

Type-family TyCons never have this property, not even data type families.
It's the *instance* TyCons that do.

I hope that this change does not break anything that somehow relied
on the old (wrong) semantics.