Hint Rewrite bits_shl bits_shru bits_shr: ints.

Lemma bitwise_binop_shl:
forall f f' x y n,
(forall x y i, 0 <= i < zwordsize -> testbit (f x y) i = f' (testbit x i) (testbit y i)) ->
f' false false = false ->
f (shl x n) (shl y n) = shl (f x y) n.