selftests/bpf: Test cross-sign 64bits range refinement

This patch adds coverage for the new cross-sign 64bits range refinement
logic. The three tests cover the cases when the u64 and s64 ranges
overlap (1) in the negative portion of s64, (2) in the positive portion
of s64, and (3) in both portions.

The first test is a simplified version of a BPF program generated by
syzkaller that caused an invariant violation [1]. It looks like
syzkaller could not extract the reproducer itself (and therefore didn't
report it to the mailing list), but I was able to extract it from the
console logs of a crash.

The principle is similar to the invariant violation described in
commit 6279846b9b25 ("bpf: Forget ranges when refining tnum after
JSET"): the verifier walks a dead branch, uses the condition to refine
ranges, and ends up with inconsistent ranges. In this case, the dead
branch is when we fallthrough on both jumps. The new refinement logic
improves the bounds such that the second jump is properly detected as
always-taken and the verifier doesn't end up walking a dead branch.

The second and third tests are inspired by the first, but rely on
condition jumps to prepare the bounds instead of ALU instructions. An
R10 write is used to trigger a verifier error when the bounds can't be
refined.

Link: https://syzkaller.appspot.com/bug?extid=c711ce17dd78e5d4fdcf [1]
Acked-by: Eduard Zingerman <eddyz87@gmail.com>
Signed-off-by: Paul Chaignon <paul.chaignon@gmail.com>
Link: https://lore.kernel.org/r/a0e17b00dab8dabcfa6f8384e7e151186efedfdd.1753695655.git.paul.chaignon@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

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