Optimize vectorized compound_indexed_add (#673)

* Optimize "none" index_constraint specialization of compound_indexed_add, so that it only reads/writes each distinct memory index once per vector.

Related to issue #637.

include/arbor/simd/simd.hpp

@@ -220,9 +220,15 @@ namespace simd_detail {
                     scalar_type a[width];
                     Impl::copy_to(s, a);
 
-                    for (unsigned i = 0; i<width; ++i) {
-                            p[o[i]] += a[i];
+                    scalar_type temp = 0;
+                    for (unsigned i = 0; i<width-1; ++i) {
+                        temp += a[i];
+                        if (o[i] != o[i+1]) {
+                            p[o[i]] += temp;
+                            temp = 0;
+                        }
                     }
+                    p[o[width-1]] = temp;
                 }
                 break;
             case index_constraint::independent: