bpf: compute instructions postorder per subprogram

The next patch would require doing postorder traversal of individual
subprograms. Facilitate this by moving env->cfg.insn_postorder
computation from check_cfg() to a separate pass, as check_cfg()
descends into called subprograms (and it needs to, because of
merge_callee_effects() logic).

env->cfg.insn_postorder is used only by compute_live_registers(),
this function does not track cross subprogram dependencies,
thus the change does not affect it's operation.

Signed-off-by: Eduard Zingerman <eddyz87@gmail.com>
Link: https://lore.kernel.org/r/20250918-callchain-sensitive-liveness-v3-5-c3cd27bacc60@gmail.com


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