Allow both INLINE and INLINABLE for the same function

Sometimes you really want both. Here is a small example:

module T where

foo :: Num a => a -> a -> a
foo x y = x+y+1

module U where

import T

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

bar :: Int -> Int -> Int
bar x y = appl foo x y

If I mark foo as INLINE, then GHC generates this code for bar:

bar1 :: Int -> Int -> Int
bar1 = foo @ Int $fNumInt

bar :: Int -> Int -> Int
bar = \ (x_aa0 :: Int) (y_aa1 :: Int) -> appl @ Int @ (Int -> Int) bar1 x_aa0 y_aa1

Whereas with INLINABLE, we get a nice specialisation but, of course, not guarantees with respect to inlining.

In general, it seems that requiring a function to inline when it is saturated and requiring it to specialise when it isn't are two different things and shouldn't be mutually exclusive.