unique vectorization

[AlexGuteniev]

Not really unique, modelled on #4987

⏬ Double load

To compare adjacent values, the same memory is loaded twice with an element shift.

It is possible to reuse the previous vector part, and mix it with the current, to save one load, but have some extra instructions to mix values, and a loop-carried dependency. On SSE path it is possible with _mm_alignr_epi8 (except for 8-bit elements). For AVX it would be way more complex due to AVX lanes.

Benchmarking shows that double load is faster than any reuse attempt. To some extent such a result overlaps with #4958

⏱️ Benchmark results

Benchmark	main	this
u<alg_type::std_fn, std::uint8_t>	1166 ns	190 ns
u<alg_type::std_fn, std::uint16_t>	1222 ns	247 ns
u<alg_type::std_fn, std::uint32_t>	1555 ns	310 ns
u<alg_type::std_fn, std::uint64_t>	1470 ns	665 ns
u<alg_type::rng, std::uint8_t>	1230 ns	187 ns
u<alg_type::rng, std::uint16_t>	1204 ns	233 ns
u<alg_type::rng, std::uint32_t>	1268 ns	308 ns
u<alg_type::rng, std::uint64_t>	1505 ns	665 ns

[CaseyCarter]

Not really unique, modelled on #4987

🤦