[llvm] [LoopInterchange] Also look at lcssa phis in outer loop latch block (PR #160889)

[Ryotaro Kasuga via llvm-commits]

https://github.com/kasuga-fj commented:

> @kasuga-fj We cannot just bail out in `moveLCSSAPhis`, when some changes have aleady been applied. If we check beforehand, we can also just remove the redundant PHI.

I was imagining an implementation that adds an additional check (or replaces redundant PHIs) to `tightlyNested`, since you also mentioned a PHI which refers to itself. That said, the current approach also seems reasonable.

https://github.com/llvm/llvm-project/pull/160889