Fix emulated arch interaction with avx512

It is possible to have a batch targeting an emulated build living
alongside a batch for arch512. In such a configuration, trying to
swizzle the emulated build will lead to instantiation of avx512 swizzle
which itself tries to instantiate a batch constant that's invalid in
avx512 terms.

Fix the situation by aggregating all avx512 swizzle of uint16_t under
one function and then switch on patterns.

Based on an idea from @hdu-sdlzx

Author: serge-sans-paille

include/xsimd/arch/xsimd_avx512f.hpp

@@ -2588,30 +2588,50 @@ namespace xsimd
                                             I16 / 2, I18 / 2, I20 / 2, I22 / 2, I24 / 2, I26 / 2, I28 / 2, I30 / 2>;
             };
 
+            template <class A, uint16_t... Is>
+            constexpr bool is_reduce_pattern()
+            {
+                // The actual pattern is {1, 1, 0, 1, 0, 1, ..., 0, 1}
+                if (sizeof...(Is) != batch<uint16_t, A>::size)
+                    return false;
+                uint16_t pattern[] = { Is... };
+                if (pattern[0] != 1)
+                    return false;
+                for (size_t i = 1; i < sizeof...(Is); i += 1)
+                {
+                    if (pattern[i] != (i & 1))
+                        return false;
+                }
+                return true;
+            }
         }
 
-        template <class A, uint16_t... Idx, class = std::enable_if_t<detail::is_pair_of_contiguous_indices<uint16_t, A, Idx...>::value>>
-        XSIMD_INLINE batch<uint16_t, A> swizzle(batch<uint16_t, A> const& self, batch_constant<uint16_t, A, Idx...>, requires_arch<avx512f>) noexcept
-        {
-            constexpr typename detail::fold_batch_constant<A, Idx...>::type mask32;
-            return _mm512_permutexvar_epi32(static_cast<batch<uint32_t, A>>(mask32), self);
-        }
-
-        template <class A>
-        XSIMD_INLINE batch<uint16_t, A>
-        swizzle(batch<uint16_t, A> const& self, batch_constant<uint16_t, A, (uint16_t)1, (uint16_t)1, (uint16_t)0, (uint16_t)1, (uint16_t)0, (uint16_t)1, (uint16_t)0, (uint16_t)1, (uint16_t)0, (uint16_t)1, (uint16_t)0, (uint16_t)1, (uint16_t)0, (uint16_t)1, (uint16_t)0, (uint16_t)1, (uint16_t)0, (uint16_t)1, (uint16_t)0, (uint16_t)1, (uint16_t)0, (uint16_t)1, (uint16_t)0, (uint16_t)1, (uint16_t)0, (uint16_t)1, (uint16_t)0, (uint16_t)1, (uint16_t)0, (uint16_t)1, (uint16_t)0, (uint16_t)1>, requires_arch<avx512f>) noexcept
+        template <class A, uint16_t... Idx>
+        XSIMD_INLINE batch<uint16_t, A> swizzle(batch<uint16_t, A> const& self, batch_constant<uint16_t, A, Idx...> mask, requires_arch<avx512f>) noexcept
         {
-            // FIXME: this sequence is very inefficient, but it's here to catch
-            // a pattern generated by detail::reduce from xsimd_common_math.hpp.
-            // The whole pattern is actually decently folded by GCC and Clang,
-            // so bare with it.
-            constexpr batch_constant<uint32_t, A, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0> mask32;
-            auto tmp = _mm512_permutexvar_epi32(static_cast<batch<uint32_t, A>>(mask32), self);
+            XSIMD_IF_CONSTEXPR(detail::is_pair_of_contiguous_indices<uint16_t, A, Idx...>::value)
+            {
+                constexpr typename detail::fold_batch_constant<A, Idx...>::type mask32;
+                return _mm512_permutexvar_epi32(static_cast<batch<uint32_t, A>>(mask32), self);
+            }
+            else XSIMD_IF_CONSTEXPR(detail::is_reduce_pattern<A, Idx...>())
+            {
+                // FIXME: this sequence is very inefficient, but it's here to catch
+                // a pattern generated by detail::reduce from xsimd_common_math.hpp.
+                // The whole pattern is actually decently folded by GCC and Clang,
+                // so bare with it.
+                constexpr batch_constant<uint32_t, A, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0> mask32;
+                auto tmp = _mm512_permutexvar_epi32(static_cast<batch<uint32_t, A>>(mask32), self);
 
-            alignas(A::alignment()) uint16_t buffer[32];
-            _mm512_store_si512((__m512i*)&buffer[0], tmp);
-            buffer[0] = buffer[1];
-            return _mm512_load_si512(&buffer[0]);
+                alignas(A::alignment()) uint16_t buffer[32];
+                _mm512_store_si512((__m512i*)&buffer[0], tmp);
+                buffer[0] = buffer[1];
+                return _mm512_load_si512(&buffer[0]);
+            }
+            else
+            {
+                return swizzle(self, mask, common {});
+            }
         }
 
         template <class A, uint16_t... Vs>