selftests: bpf: complete sub-register zero extension checks

eBPF ISA specification requires high 32-bit cleared when only low 32-bit
sub-register is written. JIT back-ends must guarantee this semantics when
doing code-gen.

This patch complete unit tests for all of those insns that could be visible
to JIT back-ends and defining sub-registers, if JIT back-ends failed to
guarantee the mentioned semantics, these unit tests will fail.

Acked-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Reviewed-by: Quentin Monnet <quentin.monnet@netronome.com>
Signed-off-by: Jiong Wang <jiong.wang@netronome.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>