Merge branch 'improve_str_concat' into optimize_mcode

mcode.cm

@@ -1576,8 +1576,10 @@ var mcode = function(ast) {
     // Standard binary ops
     left_slot = gen_expr(left, -1)
     right_slot = gen_expr(right, -1)
-    // Use target slot for ops without multi-type dispatch (add has text+num paths)
-    dest = (target >= 0 && kind != "+") ? target : alloc_slot()
+    // Use target slot for ops without multi-type dispatch (add has text+num paths).
+    // Exception: allow + to write directly to target when target == left_slot
+    // (self-assign pattern like s = s + x) since concat/add reads before writing.
+    dest = (target >= 0 && (kind != "+" || target == left_slot)) ? target : alloc_slot()
     op = binop_map[kind]
     if (op == null) {
       op = "add"