Cast worker wrapper doesn't work with INLINABLE

Consider this code, distilled from this comment in #19874 (closed).

newtype Wombat a = Wombat (a->a)

f :: Num a => Bool -> Wombat a
{-# INLINEABLE f #-}
f True  = f False
f False = Wombat (\x -> x+1)

With HEAD we get this not very nice code.

Rec {
-- RHS size: {terms: 18, types: 9, coercions: 14, joins: 0/0}
f [InlPrag=INLINABLE, Occ=LoopBreaker]
  :: forall a. Num a => Bool -> Wombat a
[GblId,
 Arity=3,
 Str=<SP(SCS(C1(L)),A,A,A,A,A,L)><1L><L>,
 Unf=Unf{Src=InlineStable, TopLvl=True, Value=True, ConLike=True,
         WorkFree=True, Expandable=True, Guidance=IF_ARGS [60 70] 240 60
         Tmpl= \ (@a_aAg)
                 ($dNum_aAh :: Num a_aAg)
                 (ds_dAv [Occ=OnceL1!] :: Bool) ->
                 (\ (eta_B0 [Occ=Once2] :: a_aAg) ->
                    case ds_dAv of {
                      False ->
                        + @a_aAg $dNum_aAh eta_B0 (fromInteger @a_aAg $dNum_aAh 1);
                      True ->
                        ((f @a_aAg $dNum_aAh GHC.Types.False)
                         `cast` (Foo.N:Wombat[0] <a_aAg>_R
                                 :: Wombat a_aAg ~R# (a_aAg -> a_aAg)))
                          eta_B0
                    })
                 `cast` (Sym (Foo.N:Wombat[0] <a_aAg>_R)
                         :: (a_aAg -> a_aAg) ~R# Wombat a_aAg)}]
f = (\ (@a_aAg)
       ($dNum_aAh :: Num a_aAg)
       (ds_dAv :: Bool)
       (eta_B0 :: a_aAg) ->
       case ds_dAv of {
         False ->
           + @a_aAg $dNum_aAh eta_B0 (fromInteger @a_aAg $dNum_aAh lvl_rAF);
         True ->
           ((f @a_aAg $dNum_aAh GHC.Types.False)
            `cast` (Foo.N:Wombat[0] <a_aAg>_R
                    :: Wombat a_aAg ~R# (a_aAg -> a_aAg)))
             eta_B0
       })
    `cast` (forall (a :: <*>_N).
            <Num a>_R
            %<'Many>_N ->_R <Bool>_R
            %<'Many>_N ->_R Sym (Foo.N:Wombat[0] <a>_R)
            :: (forall {a}. Num a => Bool -> a -> a)
               ~R# (forall {a}. Num a => Bool -> Wombat a))
end Rec }

Notice that our cast worker/wrapper stuff isn't happening. (See Note [Cast worker/wrappers] in GHC.Core.Opt.Simplify for what cast worker/wrapper is).

The problem is the stable unfolding. What we'd like to get is this:

Rec {
-- RHS size: {terms: 18, types: 9, coercions: 0, joins: 0/0}
Foo.f1 [InlPrag=INLINABLE, Occ=LoopBreaker]
  :: forall {a}. Num a => Bool -> a -> a
[GblId,
 Arity=3,
 Str=<SP(SCS(C1(L)),A,A,A,A,A,L)><1L><L>,
 Unf=Unf{Src=InlineStable, TopLvl=True, Value=True, ConLike=True,
         WorkFree=True, Expandable=True, Guidance=IF_ARGS [60 70 0] 230 0
         Tmpl= \ (@a_aAn)
                 ($dNum_aAo :: Num a_aAn)
                 (ds_dAC [Occ=Once1!] :: Bool)
                 (eta_B0 [Occ=Once2] :: a_aAn) ->
                 case ds_dAC of {
                   False ->
                     + @a_aAn $dNum_aAo eta_B0 (fromInteger @a_aAn $dNum_aAo 1);
                   True -> Foo.f1 @a_aAn $dNum_aAo GHC.Types.False eta_B0
                 }}]
Foo.f1
  = \ (@a_aAn)
      ($dNum_aAo :: Num a_aAn)
      (ds_dAC :: Bool)
      (eta_B0 :: a_aAn) ->
      case ds_dAC of {
        False ->
          + @a_aAn $dNum_aAo eta_B0 (fromInteger @a_aAn $dNum_aAo lvl_rAO);
        True -> Foo.f1 @a_aAn $dNum_aAo GHC.Types.False eta_B0
      }
end Rec }

-- RHS size: {terms: 1, types: 0, coercions: 12, joins: 0/0}
f :: forall a. Num a => Bool -> Wombat a
[GblId,
 Arity=3,
 Str=<SP(SCS(C1(L)),A,A,A,A,A,L)><1L><L>,
f = Foo.f1
    `cast` (forall (a :: <*>_N).
            <Num a>_R
            %<'Many>_N ->_R <Bool>_R
            %<'Many>_N ->_R Sym (Foo.N:Wombat[0] <a>_R)
            :: (forall {a}. Num a => Bool -> a -> a)
               ~R# (forall {a}. Num a => Bool -> Wombat a))

That is, do cast worker/wrapper, and move the stable unfolding to the worker.