[LT] Add ir_util for ComputePostOrder (#67282)
Summary: Pull Request resolved: https://github.com/pytorch/pytorch/pull/67282
Test Plan: `build/bin/test_lazy`
Reviewed By: wconstab, ngimel
Differential Revision: D31961754
Pulled By: desertfire
fbshipit-source-id: 28466588ece8057640a7202b8c79cc1a4357d373
