Skip unnecessary coverage checking

If the last clause in a definition contains only _, we don't need to
coverage check it. This vastly, vastly improves the speed of this defn:

fourTupleElems : TT -> Maybe (TT, (TT, (TT, TT)))
fourTupleElems (App (App (App (App p ty1) ty2) tm1)
                (App (App (App (App p ty3) ty4) tm2)
                 (App (App (App (App p ty1) ty2) tm3) tm4))) = Just (tm1, tm2, tm3, tm4)
fourTupleElems _ = Nothing
Merge pull request #960 from david-christiansen/skip-coverage-on-default-pattern

Skip obviously  unnecessary coverage checking