Add get_high/get_low on opcounted Vec8d

dune/perftool/common/vectorclass.hh

@@ -699,6 +699,24 @@ struct Vec8d
     BARRIER;
   }
 
+  Vec4d get_low() const
+  {
+    BARRIER;
+    Vec4d ret;
+    ret.load(&d);
+    BARRIER;
+    return ret;
+  }
+
+  Vec4d get_low() const
+  {
+    BARRIER;
+    Vec4d ret;
+    ret.load(&d + 4);
+    BARRIER;
+    return ret;
+  }
+
   Vec8d& load(const F* p)
   {
     BARRIER;