`algorithms/gcd/matrix`: Improve

[github-actions[bot]]

On 2022-06-11 @recmo wrote in e80cbc5 “Merge pull request #136 from recmo/small-devex”:

Improve

    ///
    /// Panics if `a0` does not have the highest bit set.
    /// Panics if `a0 < a1`.
    #[must_use]
    #[allow(clippy::redundant_else)]
    #[allow(clippy::cognitive_complexity)] // REFACTOR: Improve
    pub fn from_u64_prefix(a0: u64, mut a1: u64) -> Self {
        const LIMIT: u64 = 1_u64 << 32;
        debug_assert!(a0 >= 1_u64 << 63);
        debug_assert!(a0 >= a1);

From src/algorithms/gcd/matrix.rs:167