selftests/bpf: verify jset handling in CFG computation

selftests/bpf: verify jset handling in CFG computation

A test case to check if both branches of jset are explored when
computing program CFG.

At 'if r1 & 0x7 ...':
- register 'r2' is computed alive only if jump branch of jset
  instruction is followed;
- register 'r0' is computed alive only if fallthrough branch of jset
  instruction is followed.

Signed-off-by: Eduard Zingerman <eddyz87@gmail.com>
Link: https://lore.kernel.org/r/20250613175331.3238739-2-eddyz87@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>