[test] Simplify shuffle_test while pleasing gcc static analysis

The extra initialization is redundant with the call to std::fill, but
GCC 13 seems to ignore it. And it's just tests so not performance
critical.

Author: serge-sans-paille

test/test_shuffle.cpp

@@ -620,13 +620,13 @@ struct shuffle_test
     void transpose()
     {
         B b_lhs = B::load_unaligned(lhs.data());
-        std::array<B, size> b_matrix;
-        for (size_t i = 0; i < size; ++i)
-            b_matrix[i] = b_lhs;
-        std::array<value_type, size * size> ref_matrix;
-        for (size_t i = 0; i < size; ++i)
-            for (size_t j = 0; j < size; ++j)
-                ref_matrix[i * size + j] = lhs[i];
+        std::array<B, size> b_matrix = {}; // FIXME: useless but pleases gcc 13
+        std::fill(b_matrix.begin(), b_matrix.end(), b_lhs);
+
+        std::array<value_type, size * size> ref_matrix = {}; // FIXME: useless but pleases gcc 13
+        auto ref_matrix_iter = ref_matrix.begin();
+        for (size_t i = 0; i < size; ++i, ref_matrix_iter += size)
+            std::fill(ref_matrix_iter, ref_matrix_iter + size, lhs[i]);
 
         INFO("transpose");
         xsimd::transpose(b_matrix.data(), b_matrix.data() + b_matrix.size());