permute(T, idx_perm) overload

[mattkretz]

We need to consider an overload like

  template <std::size_t N = 0, vir::vectorizable T, detail::index_permutation_function<V> F>
    constexpr
    std::conditional_t<N <= 1, T, stdx::resize_simd_t<N, stdx::simd<T>>>
    permute(T const& v, F const idx_perm) noexcept

I.e. take a non-simd but vectorizable scalar + index permutation lambda. Produce a scalar or possibly a simd.

Rationale: generic code?

[mattkretz]

Hmm, I suspect LEWG will ask for a rename to simd_permute unless we make it even more generic? Like permute(std::array, ...) etc. (e.g. permute(iota_v<std::array<int, 4>>, permutations::reverse) yields {3, 2, 1, 0} as a constant expression).
Maybe the paper should ask whether there's interest in exploring a generalization? Otherwise stick with simd<T> and possibly T.

[mattkretz]

No, let's stick with "simd permute" to avoid any ambiguity wrt the return type.