[LTC] Make ComputePostOrder accept const T pointers (#88773)
Summary:
Since `c10::ArrayRef` now support `c10::ArrayRef<const T>`, let's restore `ComputePostOrder` to accept `const Node*` again, which is more suitable for the context of the given helpers.
Test Plan:
CI.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/88773
Approved by: https://github.com/JackCaoG
