Use __builtin_shufflevector for the generic implementation of shuffle

Let the compiler do the heavy lifting when possible :-)
Fix #926

include/xsimd/arch/generic/xsimd_generic_memory.hpp

@@ -419,12 +419,22 @@ namespace xsimd
                 return select(batch_bool_constant<batch<T, A>, (Indices < bsize)...>(), x, y);
             }
 
+#if defined(__has_builtin) && __has_builtin(__builtin_shuffle_vector)
+            return __builtin_shufflevector(x.data, y.data, Indices...);
+// FIXME: my experiments show that GCC only correctly optimizes this builtin
+// starting at GCC 13, where it already has __builtin_shuffle_vector
+//
+//#elif (defined(__has_builtin) && __has_builtin(__builtin_shuffle)) || GCC >= 6
+//            typedef ITy integer_vector_type __attribute__((vector_size(sizeof(batch<ITy, A>))));
+//            return __builtin_shuffle(x.data, y.data, integer_vector_type{Indices...});
+#else
             // Use a generic_pattern. It is suboptimal but clang optimizes this
             // pretty well.
             batch<T, A> x_lane = swizzle(x, batch_constant<batch<ITy, A>, ((Indices >= bsize) ? (Indices - bsize) : Indices)...>());
             batch<T, A> y_lane = swizzle(y, batch_constant<batch<ITy, A>, ((Indices >= bsize) ? (Indices - bsize) : Indices)...>());
             batch_bool_constant<batch<T, A>, (Indices < bsize)...> select_x_lane;
             return select(select_x_lane, x_lane, y_lane);
+#endif
         }
 
         // store