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/compress.js

@@ -2395,7 +2395,7 @@ merge(Compressor.prototype, {
             if (compressor.option("dead_code") && self instanceof AST_While) {
                 var a = [];
                 extract_declarations_from_unreachable_code(compressor, self.body, a);
-                return make_node(AST_BlockStatement, self, { body: a });
+                return make_node(AST_BlockStatement, self, { body: a }).optimize(compressor);
             }
             if (self instanceof AST_Do) {
                 var has_loop_control = false;
@@ -2404,7 +2404,8 @@ merge(Compressor.prototype, {
                     if (node instanceof AST_LoopControl && tw.loopcontrol_target(node) === self)
                         return has_loop_control = true;
                 });
-                self.walk(tw);
+                var parent = compressor.parent();
+                (parent instanceof AST_LabeledStatement ? parent : self).walk(tw);
                 if (!has_loop_control) return self.body;
             }
         }
@@ -2474,7 +2475,7 @@ merge(Compressor.prototype, {
                     }));
                 }
                 extract_declarations_from_unreachable_code(compressor, self.body, a);
-                return make_node(AST_BlockStatement, self, { body: a });
+                return make_node(AST_BlockStatement, self, { body: a }).optimize(compressor);
             }
             if (cond !== self.condition) {
                 cond = make_node_from_constant(cond, self.condition).transform(compressor);
@@ -2726,9 +2727,9 @@ merge(Compressor.prototype, {
             var body = [];
             if (self.bcatch) extract_declarations_from_unreachable_code(compressor, self.bcatch, body);
             if (self.bfinally) body = body.concat(self.bfinally.body);
-            return body.length > 0 ? make_node(AST_BlockStatement, self, {
+            return make_node(AST_BlockStatement, self, {
                 body: body
-            }).optimize(compressor) : make_node(AST_EmptyStatement, self);
+            }).optimize(compressor);
         }
         return self;
     });