use auto for z2v

Author: lu1and10

src/spreadinterp.cpp

@@ -796,8 +796,8 @@ Two upsampfacs implemented. Params must match ref formula. Barnett 4/24/18 */
       static constexpr uint8_t if_odd_degree = ((nc + 1) % 2);
       static constexpr uint8_t offset_start  = tail ? w - tail : w - simd_size;
       static constexpr uint8_t end_idx       = (w + (tail > 0)) / 2;
-      const simd_type zv(z);
-      const simd_type z2v = zv * zv;
+      const simd_type zv{z};
+      const auto z2v = zv * zv;
 
       // some xsimd constant for shuffle or inverse
       static constexpr auto shuffle_batch = []() constexpr noexcept {