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

test/mocha/minify.js

@@ -182,4 +182,13 @@ describe("minify", function() {
         });
     });
 
+    describe("Compressor", function() {
+        it("should be backward compatible with ast.transform(compressor)", function() {
+            var ast = Uglify.parse("function f(a){for(var i=0;i<a;i++)console.log(i)}");
+            ast.figure_out_scope();
+            ast = ast.transform(Uglify.Compressor());
+            assert.strictEqual(ast.print_to_string(), "function f(a){for(var i=0;i<a;i++)console.log(i)}");
+        });
+    })
+
 });