selftests/bpf: Add few tests for alu32 shift value tracking and zext

Add few more alu32 shift tests using div-by-zero on provably dead paths
to check both verifier and JIT xlation resp. runtime correctness.

If the verifier mistracks the result, it rejects due to the div by 0;
if the JIT computes a wrong value, then runtime hits the dead path and
retval changes.

  # LDLIBS=-static PKG_CONFIG='pkg-config --static' ./vmtest.sh -- ./test_progs -t verifier_subreg
  [...]
  #644/76  verifier_subreg/arsh32_imm1_value:OK
  #644/77  verifier_subreg/lsh32_reg0_zero_extend_check:OK
  #644/78  verifier_subreg/rsh32_reg0_zero_extend_check:OK
  #644/79  verifier_subreg/arsh32_reg0_zero_extend_check:OK
  #644/80  verifier_subreg/lsh32_imm31_value:OK
  #644/81  verifier_subreg/rsh32_imm31_value:OK
  #644/82  verifier_subreg/arsh32_imm31_value:OK
  #644/83  verifier_subreg/lsh32_unknown_precise_bounds:OK
  #644/84  verifier_subreg/rsh32_unknown_bounds:OK
  #644     verifier_subreg:OK
  Summary: 1/84 PASSED, 0 SKIPPED, 0 FAILED

Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/r/20260327220629.343327-1-daniel@iogearbox.net
Signed-off-by: Alexei Starovoitov <ast@kernel.org>