selftests/bpf: check nullness propagation for reg to reg comparisons

Verify that nullness information is porpagated in the branches of
register to register JEQ and JNE operations.

Signed-off-by: Eduard Zingerman <eddyz87@gmail.com>
Acked-by: Yonghong Song <yhs@fb.com>
Link: https://lore.kernel.org/r/20221115224859.2452988-3-eddyz87@gmail.com


Signed-off-by: Alexei Starovoitov <ast@kernel.org>