It turns out we were using two different sets of type variables when
reifying data family instances in Template Haskell. We were using the
tyvars quantifying over the instance itself for the LHS, but using the
tyvars quantifying over the data family instance constructor for the
RHS. This commit uses the instance tyvars for both the LHS and the RHS,
fixing #13618.

Test Plan: make test TEST=T13618

Reviewers: goldfire, austin, bgamari

Reviewed By: goldfire, bgamari

Subscribers: rwbarton, thomie

GHC Trac Issues: #13618

Differential Revision: https://phabricator.haskell.org/D3505