selftests/bpf: Add test for sign extension in coerce_subreg_to_size_sx()

Add a test for unsigned ranges after signed extension instruction. This
case isn't currently covered by existing tests in verifier_movsx.c.
Acked-by: Shung-Hsi Yu <shung-hsi.yu@suse.com>
Signed-off-by: Dimitar Kanaliev <dimitar.kanaliev@siteground.com>
Acked-by: Yonghong Song <yonghong.song@linux.dev>
Link: https://lore.kernel.org/r/20241014121155.92887-4-dimitar.kanaliev@siteground.comSigned-off-by: Alexei Starovoitov <ast@kernel.org>

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

@@ -307,6 +307,26 @@ label_%=:						\
 	: __clobber_all);
 }
 
+SEC("socket")
+__description("MOV32SX, S8, unsigned range_check")
+__success __retval(0)
+__naked void mov32sx_s8_range_check(void)
+{
+	asm volatile ("                                 \
+	call %[bpf_get_prandom_u32];                    \
+	w0 &= 0x1;                                      \
+	w0 += 0xfe;                                     \
+	w0 = (s8)w0;                                    \
+	if w0 < 0xfffffffe goto label_%=;               \
+	r0 = 0;                                         \
+	exit;                                           \
+label_%=: 	                                        \
+	exit;                                           \
+	"      :
+	: __imm(bpf_get_prandom_u32)
+	: __clobber_all);
+}
+
 #else
 
 SEC("socket")