bpf: make state->dfs_depth < state->loop_entry->dfs_depth an invariant

bpf: make state->dfs_depth < state->loop_entry->dfs_depth an invariant

For a generic loop detection algorithm a graph node can be a loop
header for itself. However, state loop entries are computed for use in
is_state_visited(), where get_loop_entry(state)->branches is checked.
is_state_visited() also checks state->branches, thus the case when
state == state->loop_entry is not interesting for is_state_visited().

This change does not affect correctness, but simplifies
get_loop_entry() a bit and also simplifies change to
update_loop_entry() in patch 9.

Signed-off-by: Eduard Zingerman <eddyz87@gmail.com>
Link: https://lore.kernel.org/r/20250215110411.3236773-7-eddyz87@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>