bpf: Simplify cnum normalize() by dropping unnecessary comparisons

In normalize(), there's no need to compare cnum.base with ST_MAX,
because any cnum with size == UT_MAX represents the full unsigned domain
independently of base, so normalize() can canonicalize all such values
to base 0. Also, it's a no-op assigning 0 to cnum.base if the cnum is
already unbounded, so we can remove the base != 0 check too.

We've also run some benchmarks to compare the performance of the new
implementation. We've run the tests with O2 optimization.

Results:

        | Test                    | normalize   | normalize_new |
        | ----------------------- | ----------- | ------------- |
        | normal small range      | 1.255 ns/op | 1.217 ns/op   |
        | full range normalized   | 1.019 ns/op | 1.217 ns/op   |
        | full range nonzero base | 1.031 ns/op | 1.221 ns/op   |
        | empty                   | 1.023 ns/op | 1.217 ns/op   |
        | signed max full range   | 1.020 ns/op | 1.218 ns/op   |
        | max base singleton      | 1.218 ns/op | 1.219 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
the operation 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/20260730102129.15209-3-vldsampaio@pm.me
Signed-off-by: Eduard Zingerman <eddyz87@gmail.com>

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