Add fallback methods for zip_hi and zip_lo

Change-Id: If2aca405292017839dffa2800f900038247d1ad5
Signed-off-by: Yuqi Gu <guyuqi@apache.com>

Author: Yuqi Gu

include/xsimd/types/xsimd_fallback.hpp

@@ -928,6 +928,28 @@ namespace xsimd
             {
                 XSIMD_FALLBACK_MAPPING_LOOP(batch_bool, std::isnan(x[i]))
             }
+
+            static batch_type zip_lo(const batch_type& lhs, const batch_type& rhs)
+            {
+                batch_type b_lo;
+                for (std::size_t i = 0, j = 0; i < N/2; ++i, j = j + 2)
+                {
+                    b_lo[j] = lhs[i];
+                    b_lo[j + 1] = rhs[i];
+                }
+                return b_lo;
+            }
+
+            static batch_type zip_hi(const batch_type& lhs, const batch_type& rhs)
+            {
+                batch_type b_hi;
+                for (std::size_t i = 0, j = 0; i < N/2; ++i, j = j + 2)
+                {
+                    b_hi[j] = lhs[i + N/2];
+                    b_hi[j + 1] = rhs[i+ N/2];
+                }
+                return b_hi;
+            }
         };
     }
 

test/test_shuffle_128.cpp

@@ -62,12 +62,12 @@ class shuffle_128_test : public testing::Test
 
     void shuffle_128_low_high()
     {
-        auto shuffle_base = xsimd::init_shuffle_128_base<value_type, size>{};
+        xsimd::init_shuffle_128_base<value_type, size> shuffle_base;
         auto shuffle_base_vecs = shuffle_base.create_vectors();
-	    auto v_lhs = shuffle_base_vecs[0];
-	    auto v_rhs = shuffle_base_vecs[1];
-	    auto v_exp_lo = shuffle_base_vecs[2];
-	    auto v_exp_hi = shuffle_base_vecs[3];
+        auto v_lhs = shuffle_base_vecs[0];
+        auto v_rhs = shuffle_base_vecs[1];
+        auto v_exp_lo = shuffle_base_vecs[2];
+        auto v_exp_hi = shuffle_base_vecs[3];
 
         B b_lhs, b_rhs, b_exp_lo, b_exp_hi, b_res_lo, b_res_hi;
         b_lhs.load_unaligned(v_lhs.data());