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

[Michael Kruse via llvm-commits]

https://github.com/Meinersbur commented:

If LoopInterchange has to build LCSSA form again anyway, have you considered cleaning this up at the same time? Then to be on the save side, do not any PHI nodes in `OuterLoopLatch` unless `InnerLoopExit == OuterLoopLatch` (like there is already a check `InnerLoopPreHeader != OuterLoopHeader` in `tightlyNested`). In addition to the case where `%old.cond.lcssa` is not trivally dead, I could imagine other dubious patterns, such as a PHI node referencing itself.

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