bpf: Avoid redundant min()/max() in cnum signed bounds

Simplify the smin() and smax() calculation by removing the integer min
and max calculation on the non-overflowing branch. This is a valid
simplification because when the arc does not cross the signed
ST_MAX/ST_MIN boundary, the signed order of the endpoints is preserved,
so base is the signed minimum and base + size is the signed maximum.
Therefore, the min()/max() calls are redundant.

We've also run benchmarks to compare the performance of this change.
With O2 optimization we have:

smin() results:

        | Test                     | smin        | smin_new    |
        | ------------------------ | ----------- | ----------- |
        | empty                    | 1.866 ns/op | 1.477 ns/op |
        | positive range           | 1.870 ns/op | 1.248 ns/op |
        | negative range           | 1.657 ns/op | 1.246 ns/op |
        | cross zero               | 1.870 ns/op | 1.660 ns/op |
        | signed overflow boundary | 1.463 ns/op | 1.049 ns/op |
        | signed overflow range    | 1.519 ns/op | 1.050 ns/op |
        | unsigned overflow range  | 1.883 ns/op | 1.669 ns/op |
        | full range               | 1.555 ns/op | 1.049 ns/op |

smax() results:

        | Test                     | smax        | smax_new    |
        | ------------------------ | ----------- | ----------- |
        | empty                    | 1.462 ns/op | 1.456 ns/op |
        | positive range           | 1.250 ns/op | 1.249 ns/op |
        | negative range           | 1.259 ns/op | 1.297 ns/op |
        | cross zero               | 1.471 ns/op | 1.475 ns/op |
        | signed overflow boundary | 1.056 ns/op | 1.260 ns/op |
        | signed overflow range    | 1.049 ns/op | 1.259 ns/op |
        | unsigned overflow range  | 1.470 ns/op | 1.477 ns/op |
        | full range               | 1.072 ns/op | 1.263 ns/op |

You can check the full implementation of these tests on this repository:

        https://github.com/OpenSourceVerif/open-verified-artifacts/tree/cnum-refactor-verif/cnum/benches

Lean4 proofs showing that the new implementation preserves soundness of
these operations are available at:

        https://github.com/OpenSourceVerif/open-verified-artifacts/tree/cnum-refactor-verif/cnum/lean

Co-developed-by: Yazhou Tang <tangyazhou518@outlook.com>
Co-developed-by: Shenghao Yuan <shenghaoyuan0928@163.com>
Signed-off-by: Yazhou Tang <tangyazhou518@outlook.com>
Signed-off-by: Shenghao Yuan <shenghaoyuan0928@163.com>
Signed-off-by: Vinicius Sampaio <vldsampaio@pm.me>
Acked-by: Shung-Hsi Yu <shung-hsi.yu@suse.com>
Link: https://patch.msgid.link/20260730102129.15209-4-vldsampaio@pm.me
Signed-off-by: Eduard Zingerman <eddyz87@gmail.com>

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