Add Vec8d(Vec4d, Vec4d) constructor

dune/perftool/common/vectorclass.hh

@@ -693,6 +693,14 @@ struct Vec8d
     BARRIER;
   }
 
+  Vec8d(Vec4d low, Vec4d high)
+  {
+    BARRIER;
+    std::copy(_d, _d+4, low._d);
+    std::copy(_d+4, _d+8, high._d);
+    BARRIER;
+  }
+
   Vec8d(F d0, F d1, F d2, F d3, F d4, F d5, F d6, F d7)
     : _d{d0,d1,d2,d3,d4,d5,d6,d7}
   {