Merge branch 'optimize_concat' of https://github.com/rvanvelzen/UglifyJS2 into rvanvelzen-optimize_concat

lib/compress.js

@@ -1656,6 +1656,11 @@ merge(Compressor.prototype, {
               case "<=": reverse(">="); break;
             }
         }
+        if (self.operator == "+" && self.right instanceof AST_String
+            && self.right.getValue() === "" && self.left instanceof AST_Binary
+            && self.left.operator == "+" && self.left.is_string()) {
+            return self.left;
+        }
         return self;
     });