`algorithms/gcd/matrix`: Improve

*On 2022-06-11 @recmo wrote in [`e80cbc5`](https://github.com/recmo/uint/commit/e80cbc559c54ff9cba55e24129faf23e89933ee8) “Merge pull request #136 from recmo/small-devex”:*

Improve

```rust
///
/// 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`](https://github.com/recmo/uint/blob/e80cbc559c54ff9cba55e24129faf23e89933ee8/src/algorithms/gcd/matrix.rs#L167)*