bpf: table based bpf_insn_successors()

Converting bpf_insn_successors() to use lookup table makes it ~1.5
times faster.

Also remove unnecessary conditionals:
- `idx + 1 < prog->len` is unnecessary because after check_cfg() all
  jump targets are guaranteed to be within a program;
- `i == 0 || succ[0] != dst` is unnecessary because any client of
  bpf_insn_successors() can handle duplicate edges:
  - compute_live_registers()
  - compute_scc()

Moving bpf_insn_successors() to liveness.c allows its inlining in
liveness.c:__update_stack_liveness().
Such inlining speeds up __update_stack_liveness() by ~40%.
bpf_insn_successors() is used in both verifier.c and liveness.c.
perf shows such move does not negatively impact users in verifier.c,
as these are executed only once before main varification pass.
Unlike __update_stack_liveness() which can be triggered multiple
times.

Signed-off-by: Eduard Zingerman <eddyz87@gmail.com>
Link: https://lore.kernel.org/r/20250918-callchain-sensitive-liveness-v3-10-c3cd27bacc60@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

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