bpf: verifier: mark patched-insn with sub-register zext flag

Patched insns do not go through generic verification, therefore doesn't has
zero extension information collected during insn walking.

We don't bother analyze them at the moment, for any sub-register def comes
from them, just conservatively mark it as needing zero extension.

Signed-off-by: Jiong Wang <jiong.wang@netronome.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>