selftests/bpf: BPF test_verifier selftests for bpf_loop inlining

A number of test cases for BPF selftests test_verifier to check how
bpf_loop inline transformation rewrites the BPF program. The following
cases are covered:
 - happy path
 - no-rewrite when flags is non-zero
 - no-rewrite when callback is non-constant
 - subprogno in insn_aux is updated correctly when dead sub-programs
   are removed
 - check that correct stack offsets are assigned for spilling of R6-R8
   registers

Signed-off-by: Eduard Zingerman <eddyz87@gmail.com>
Acked-by: Song Liu <songliubraving@fb.com>
Link: https://lore.kernel.org/r/20220620235344.569325-5-eddyz87@gmail.com


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