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