selftests/bpf: Add test for BPF_END register ID reset

selftests/bpf: Add test for BPF_END register ID reset

Add a test case to ensure that BPF_END operations correctly break
register's scalar ID ties.

The test creates a scenario where r1 is a copy of r0, r0 undergoes a
byte swap, and then r0 is checked against a constant.

- Without the fix in the verifier, the bounds learned from r0 are
  incorrectly propagated to r1, making the verifier believe r1 is
  bounded and wrongly allowing subsequent pointer arithmetic.

- With the fix, r1 remains an unbounded scalar, and the verifier
  correctly rejects the arithmetic operation between the frame pointer
  and the unbounded register.

Co-developed-by: Tianci Cao <ziye@zju.edu.cn>
Signed-off-by: Tianci Cao <ziye@zju.edu.cn>
Co-developed-by: Shenghao Yuan <shenghaoyuan0928@163.com>
Signed-off-by: Shenghao Yuan <shenghaoyuan0928@163.com>
Signed-off-by: Yazhou Tang <tangyazhou518@outlook.com>
Acked-by: Eduard Zingerman <eddyz87@gmail.com>
Link: https://lore.kernel.org/r/20260304083228.142016-3-tangyazhou@zju.edu.cn
Signed-off-by: Alexei Starovoitov <ast@kernel.org>