selftests/bpf: test cases for bpf_loop SCC and state graph backedges

selftests/bpf: test cases for bpf_loop SCC and state graph backedges

Test for state graph backedges accumulation for SCCs formed by
bpf_loop(). Equivalent to the following C program:

  int main(void) {
    1: fp[-8] = bpf_get_prandom_u32();
    2: fp[-16] = -32;                       // used in a memory access below
    3: bpf_loop(7, loop_cb4, fp, 0);
    4: return 0;
  }

  int loop_cb4(int i, void *ctx) {
    5: if (unlikely(ctx[-8] > bpf_get_prandom_u32()))
    6:   *(u64 *)(fp + ctx[-16]) = 42;      // aligned access expected
    7: if (unlikely(fp[-8] > bpf_get_prandom_u32()))
    8:   ctx[-16] = -31;                    // makes said access unaligned
    9: return 0;
  }

If state graph backedges are not accumulated properly at the SCC
formed by loop_cb4() call from bpf_loop(), the state {ctx[-16]=-32}
injected at instruction 9 on verification path 1,2,3,5,7,9,4 would be
considered fully verified and would lack precision mark for ctx[-16].
This would lead to early pruning of verification path 1,2,3,5,7,8,9 in
state {ctx[-16]=-31}, which in turn leads to the incorrect assumption
that the above program is safe.

Signed-off-by: Eduard Zingerman <eddyz87@gmail.com>
Link: https://lore.kernel.org/r/20251229-scc-for-callbacks-v1-2-ceadfe679900@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>