Optimize reverse for 8 and 16-bit trivial types

[Alex Guteniev (AlexGuteniev)]

Faster reverse tail.

The benchmark uses random lengths:

#include <cstdint>
#include <chrono>
#include <iostream>
#include <random>

constexpr std::size_t N = 2048;
constexpr std::size_t NS = 8192;
constexpr std::size_t R = 10'000;

alignas(64) std::uint8_t    a8[N];
alignas(64) std::uint16_t   a16[N / 2];
alignas(64) std::uint32_t   a32[N / 4];
alignas(64) std::uint64_t   a64[N / 8];

alignas(64) std::uint8_t    d8[N];
alignas(64) std::uint16_t   d16[N / 2];
alignas(64) std::uint32_t   d32[N / 4];
alignas(64) std::uint64_t   d64[N / 8];

template<typename T, std::size_t S>
void rev(bool c, T(&a)[S], T(&d)[S], const char* name) {
    std::mt19937 gen(65521);
    std::uniform_int_distribution<std::size_t> dis(0, S);
    std::size_t sizes[NS];
    for (auto& s : sizes) {
        s = dis(gen);
    }

    auto t1 = std::chrono::steady_clock::now();
    if (c) {
        for (std::size_t i = 0; i < R; i++) {
            for (std::size_t s = 0; s < NS; s++) {
                std::reverse_copy(std::begin(a), std::begin(a) + sizes[s], std::begin(d));
            }
        }
    }
    else {
        for (std::size_t i = 0; i < R; i++) {
            for (std::size_t s = 0; s < NS; s++) {
                std::reverse(std::begin(a), std::begin(a) + sizes[s]);
            }
        }
    }
    auto t2 = std::chrono::steady_clock::now();
    std::cout << name << ":\t" << std::chrono::duration_cast<std::chrono::duration<double>>(t2 - t1).count() << "s\n";
}

int main()
{
    rev(false, a8, d8, "reverse 8");
    rev(false, a16, d16, "reverse 16");
    rev(false, a32, d32, "reverse 32");
    rev(false, a64, d64, "reverse 64");
    rev(true, a8, d8, "rev. copy 8");
    rev(true, a16, d16, "rev. copy 16");
    rev(true, a32, d32, "rev. copy 32");
    rev(true, a64, d64, "rev. copy 64");
}

On my Intel(R) Core(TM) i7-8750H CPU @ 2.20GHz

Name	Before	After	Before JCC mtg	After JCC mtg
reverse 8	3.46938s	2.45349s	2.74506s	2.41771s
reverse 16	2.91174s	2.49834s	2.5235s	2.36707s
reverse 32	2.44931s	2.46176s	2.4739s	2.50781s
reverse 64	2.18105s	2.32562s	2.21836s	2.22949s
rev. copy 8:	2.80264s	2.4998s	2.81799s	2.51851s
rev. copy 16	2.83642s	2.4392s	2.71336s	2.43827s
rev. copy 32	2.56736s	2.71499s	2.46718s	2.468s
rev. copy 64	2.21659s	2.32269s	2.21912s	2.21747s

JCC mtg = added /QIntel-jcc-erratum to root makefile

[Alex Guteniev (AlexGuteniev)]

The results are small and ambiguous, I'm afraid I cannot prove this PR is much helpful.
If anyone disagrees and has strong confidence in this direction, feel free to pick it up.