Make better use of known dictionaries

Example:

{-# LANGUAGE GADTs #-}
module T5835 where

data T a where
  T :: Eq a => a -> T a

foo :: a -> T a -> Bool
{-# INLINE foo #-}
foo x = \(T y) -> x == y

appl :: (a -> b) -> a -> b
{-# NOINLINE appl #-}
appl f x = f x

bar :: T Int -> Bool
bar t = appl (foo 42) t

GHC generates this for bar:

bar2 :: Int
bar2 = I# 42

bar1 :: T Int -> Bool
bar1 =
  \ (ds_dhk :: T Int) ->
    case ds_dhk of _ { T $dEq_agz y_aa4 ->
    == @ Int $dEq_agz bar2 y_aa4
    }

bar :: T Int -> Bool
bar = \ (t_aga :: T Int) -> appl @ (T Int) @ Bool bar1 t_aga

Note how it want to get the Eq dictionary for Int from T. But we know the Eq Int instance without inspecting T and bar could be significantly simplified if we used that.