There was a special case in rewriteEvidence, looking like:
  = return (Just (if ctEvPred old_ev `tcEqType` new_pred
                  then old_ev
                  else old_ev { ctev_pred = new_pred }))

But this was wrong: old_pred and new_pred might differ in the kind
of a TyVar occurrence, in which case tcEqType would not notice,
but we really, really want new_pred.  This caused Trac #8913.

I solved this by dropping the whole test, and instead making
the flattener preserve type synonyms. This was easy because
TcEvidence has TcTyConAppCo which (unlike) Coercion, handles
synonyms.