Hint Rewrite bits_shl bits_shru bits_shr: ints.

Lemma bitwise_binop_shru:
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 (shru x n) (shru y n) = shru (f x y) n.