Commit 193664d4 added a
special caseRule for `dataToTag`, but this transformation completely
broke when `dataToTag` was applied to somewith with a type headed by
a data family, leading to #14680. For now at least, the simplest
solution is to simply not apply this transformation when the type is
headed by a data family.

Test Plan: make test TEST=T14680

Reviewers: simonpj, bgamari

Reviewed By: bgamari

Subscribers: rwbarton, thomie, carter

GHC Trac Issues: #14680

Differential Revision: https://phabricator.haskell.org/D4371