Fix assert trap in schedule_to_source()
Fix assert trap in schedule_to_source()

Don’t call LoopLevel::var() unless we know is_root is false


Former-commit-id: 554c80d5877a1068a4fed6af591dffd46fb20ceb
Trass3r/Halide,darkbuck/Halide,darkbuck/Halide,darkbuck/Halide,darkbuck/Halide,darkbuck/Halide,darkbuck/Halide,darkbuck/Halide,Trass3r/Halide,Trass3r/Halide,Trass3r/Halide,Trass3r/Halide,Trass3r/Halide,Trass3r/Halide