Hint Rewrite @sem_add_ptr_int_lem using assumption : norm1.

Lemma sem_add_pi': forall {CS: compspecs} t0 si v i,
isptr v ->
match si with
| Signed => Int.min_signed <= i <= Int.max_signed
| Unsigned => 0 <= i <= Int.max_unsigned
end ->
force_val (sem_add_ptr_int t0 si v (Vint (Int.repr i))) =
offset_val (sizeof t0 * i) v.