selftests, bpf: Test that dead ldx_w insns are accepted

Prevent regressions related to zero-extension metadata handling during
dead code sanitization.
Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20210812151811.184086-3-iii@linux.ibm.com

tools/testing/selftests/bpf/verifier/dead_code.c

@@ -159,3 +159,15 @@
 	.result = ACCEPT,
 	.retval = 2,
 },
+{
+	"dead code: zero extension",
+	.insns = {
+	BPF_MOV64_IMM(BPF_REG_0, 0),
+	BPF_STX_MEM(BPF_W, BPF_REG_10, BPF_REG_0, -4),
+	BPF_JMP_IMM(BPF_JGE, BPF_REG_0, 0, 1),
+	BPF_LDX_MEM(BPF_W, BPF_REG_0, BPF_REG_10, -4),
+	BPF_EXIT_INSN(),
+	},
+	.result = ACCEPT,
+	.retval = 0,
+},