[LoopUnrollAndJam] Visit phi operand dependencies in post-order
Fixes https://github.com/llvm/llvm-project/issues/58565
The previous implementation visits operands in pre-order, but this does
not guarantee an instruction is visited before its uses. This can cause
instructions to be copied in the incorrect order. For example:
```
a = ...
b = add a, 1
c = add a, b
d = add b, a
```
Pre-order visits does not guarantee the order in which `a` and `b` are
visited. LoopUnrollAndJam may incorrectly insert `b` before `a`.
This patch implements post-order visits. By visiting dependencies first,
we guarantee that an instruction's dependencies are visited first.
Differential Revision: https://reviews.llvm.org/D140255