selftests/bpf: add iter test requiring range x range logic

Add a simple verifier test that requires deriving reg bounds for one
register from another register that's not a constant. This is
a realistic example of iterating elements of an array with fixed maximum
number of elements, but smaller actual number of elements.

This small example was an original motivation for doing this whole patch
set in the first place, yes.
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/r/20231112010609.848406-14-andrii@kernel.orgSigned-off-by: Alexei Starovoitov <ast@kernel.org>

tools/testing/selftests/bpf/progs/iters.c

@@ -1411,4 +1411,26 @@ __naked int checkpoint_states_deletion(void)
 	);
 }
 
+struct {
+	int data[32];
+	int n;
+} loop_data;
+
+SEC("raw_tp")
+__success
+int iter_arr_with_actual_elem_count(const void *ctx)
+{
+	int i, n = loop_data.n, sum = 0;
+
+	if (n > ARRAY_SIZE(loop_data.data))
+		return 0;
+
+	bpf_for(i, 0, n) {
+		/* no rechecking of i against ARRAY_SIZE(loop_data.n) */
+		sum += loop_data.data[i];
+	}
+
+	return sum;
+}
+
 char _license[] SEC("license") = "GPL";