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

[Sjoerd Meijer via llvm-commits]

sjoerdmeijer wrote:

I think I've addressed all comments. I've introduced a new helper that is now more intentional and explicit about checking non-exit blocks and single-input phis. I've added an assert there that I haven't managed to trigger in testing. I have changed the test case and made that a bit more interesting: the problematic phi was trivially dead before, but now it has a user in the exit block, which shows that we simplify the phi-chain and replace all uses. 

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