Proof with (try discriminate).
intros. unfold is_longconst in *. destruct Archi.splitlong.
eapply SplitLongproof.is_longconst_sound; eauto.
assert (a = Eop (Olongconst n) Enil).
{ destruct a... destruct o... destruct e0... congruence. }
subst a. InvEval. auto.
Qed.

Theorem eval_cmpl:
forall c le a x b y v,
eval_expr ge sp e m le a x ->
eval_expr ge sp e m le b y ->
Val.cmpl c x y = Some v ->
eval_expr ge sp e m le (cmpl c a b) v.