Fix egregious blunder in the type flattener

In tidying up the flattener I introduced an error that no
regression test caught, giving rise to Trac #8978, #8979.
It shows up if you have a type synonym whose RHS mentions
type functions, such sas
     type family F a
     type T a = (F a, a)   -- This synonym isn't properly flattened

The fix is easy, but sadly the bug is in the released GHC 7.8.1

(cherry picked from commit b8132a9d2fdb93c5d30107b1d531dd73ac27b262)