Consider
  type family F a :: * -> *

Then (a ~ F Int a) is an insoluble occurs check, and can be reported
as such.  Previous to this patch, TcType.isInsolubleOccursCheck was
treating any type-family application (including an over-saturated one)
as unconditionally not-insoluble.

This really only affects error messages, and then only slightly. I
tripped over this when investigating  Trac #13674.