invert reduce_vars tracking flag (#1519)

Modules like webpack and grunt-contrib-uglify still uses `ast.transform(compressor)` before `Compressor.compress(ast)` was introduced.

Workaround this compatibility issue by deactivating `reduce_vars` in such case.

Also fix use case with omitted `options` when calling `Compressor()`.

fixes #1516

README.md

@@ -621,7 +621,7 @@ function uglify(ast, options, mangle) {
 
   // Compression
   uAST.figure_out_scope();
-  uAST = uAST.transform(UglifyJS.Compressor(options));
+  uAST = UglifyJS.Compressor(options).compress(uAST);
 
   // Mangling (optional)
   if (mangle) {
@@ -865,7 +865,7 @@ toplevel.figure_out_scope()
 Like this:
 ```javascript
 var compressor = UglifyJS.Compressor(options);
-var compressed_ast = toplevel.transform(compressor);
+var compressed_ast = compressor.compress(toplevel);
 ```
 
 The `options` can be missing.  Available options are discussed above in