fix label-related bugs (#1835)

- deep cloning of `AST_LabeledStatement`
- `L:do{...}while(false)`
- empty statement with label within block

extend `test/ufuzz.js`
- generate labels for blocks & loops
- generate for-in statements
- skip suspicious option search if `minify()` errs


fixes #1833

lib/ast.js

@@ -206,12 +206,13 @@ var AST_LabeledStatement = DEFNODE("LabeledStatement", "label", {
     clone: function(deep) {
         var node = this._clone(deep);
         if (deep) {
-            var refs = node.label.references;
-            var label = this.label;
+            var label = node.label;
+            var def = this.label;
             node.walk(new TreeWalker(function(node) {
                 if (node instanceof AST_LoopControl
-                    && node.label && node.label.thedef === label) {
-                    refs.push(node);
+                    && node.label && node.label.thedef === def) {
+                    node.label.thedef = label;
+                    label.references.push(node);
                 }
             }));
         }