Make worker-wrapper unbox data families

I noticed that the worker-wrapper optimization doesn't unbox arguments whose type is a data family instance. For example in this module:

{-# LANGUAGE TypeFamilies #-}
{-# LANGUAGE BangPatterns #-}
module Foo where
data family Foo a

data instance Foo Int = FooInt Int Int

foo :: Foo Int -> Int
foo (FooInt a b) = loop a b
    where
        loop 0 y = length $ replicate y b
        loop x !y = loop (mod y x) x

foo1 :: (Int, Int) -> Int
foo1 (a, b) = loop a b
    where
        loop 0 y = length $ replicate y b
        loop x !y = loop (mod y x) x

foo and foo1 both get worker-wrapper applied, with worker functions of the following types:

$wfoo :: Foo Int -> Int#
$wfoo1 :: Int# -> Int# -> Int#

It would be nice if $wfoo could get the same type as $wfoo1.

This issue happened in real life with unboxed vectors from the vector package, resulting in a lot of boxing with unboxed vector constructors immediately followed by unboxing.

Trac metadata

Trac field	Value
Version	7.7
Type	FeatureRequest
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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