selftests/bpf: iterator based loop and STACK_MISC states pruning

selftests/bpf: iterator based loop and STACK_MISC states pruning

The test case first initializes 9 stack slots as STACK_MISC,
then conditionally updates each of them to SCALAR spill inside an
iterator based loop. This leads to 2**9 combinations of MISC/SPILL
marks for these slots at the iterator next call.
The loop converges only if the verifier treats such states as
equivalent, otherwise visited states are evicted from the states cache
too quickly.

Signed-off-by: Eduard Zingerman <eddyz87@gmail.com>
Link: https://lore.kernel.org/r/20251230-loop-stack-misc-pruning-v1-2-585cfd6cec51@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>