[LoopInterchange] Fix test phi-ordering.ll (NFC) (#181989)
I found that the test phi-ordering.ll is a bit fragile and can fail with
any irrelevant changes. Also this test is not consistent with the
following comment, which is at the top of the file:
```
;; Checks the order of the inner phi nodes does not cause havoc.
;; The inner loop has a reduction into c. The IV is not the first phi.
```
After examining the change history, I found that the original intent of
this test was effectively lost in
https://github.com/llvm/llvm-project/commit/c8bd6ea35e459169cbd401372e81168ed8482536.
A workaround was introduced later in
https://github.com/llvm/llvm-project/commit/eac34875109898ac01985f4afa937eec30c1c387
to preserve the test output, but this seems to have made the test more
complicated.
This patch updates the test so that its original intent is restored. The
change is close to a partial revert of
https://github.com/llvm/llvm-project/commit/c8bd6ea35e459169cbd401372e81168ed8482536.
Even though the feature for inner-only reductions has been removed, I
believe it is still better to keep the test as it is, otherwise the test
would be meaningless.
