Proof with eauto with val_erasure.
intros.
destruct v, chunk; simpl; unfold inj_value; simpl;
repeat econstructor...
Qed.

Lemma val_erasure_longofwords:
forall v1 v1' v2 v2'
(Hval_erasure: val_erasure v1 v1')
(Hval_erasure: val_erasure v2 v2'),
val_erasure (Val.longofwords v1 v2) (Val.longofwords v1' v2').