re-enable fast isinstance

Cython/Compiler/Optimize.py

@@ -162,7 +162,7 @@ class FinalOptimizePhase(Visitor.CythonTransform):
 
     def visit_SimpleCallNode(self, node):
         self.visitchildren(node)
-        if 0 and node.function.type.is_cfunction and isinstance(node.function, ExprNodes.NameNode):
+        if node.function.type.is_cfunction and isinstance(node.function, ExprNodes.NameNode):
             if node.function.name == 'isinstance':
                 type_arg = node.args[1]
                 if type_arg.type.is_builtin_type and type_arg.type.name == 'type':