Don't inline UDFs with non-cheap children in CollapseProject

Because UDFs (and certain other expressions) are considered cheap by CollapseProject.isCheap, they are inlined and potentially duplicated (which is ok, because rules like ExtractPythonUDFs will de-duplicate them). However, if the UDFs contain other non-cheap expressions, those will also be duplicated and can potentially cause performance regressions.