Workaround failed constant-folding for zeroBits

For some reason GHC fails to constant fold `zeroBits :: Int` and
`zeroBits :: Integer`; `ghc -show-iface` shows

  $fBitsInt_$czeroBits :: GHC.Types.Int
     {- Strictness: m,
        Unfolding: (GHC.Types.I# (GHC.Prim.andI# 1 (GHC.Prim.notI# 1))) -}

Otoh, constant-folding works as expected, reducing `zeroBits` to 0 constant
for the other integer-types (= {Word,Int}{8,16,32,64}` and `Word`). So this
quickfix is actually just treating the symptom rather than the cause.

Signed-off-by: Herbert Valerio Riedel <hvr@gnu.org>