[llvm] [LoopInterchange] Swap preheader contents before rebuilding LCSSA (PR #218468)

[Madhur Amilkanthwar via llvm-commits]

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madhur13490 wrote:

I think after this line, you should assert something like below (with proper comment):

```
// Guard for a future reordering 
assert(!verifyFunction(*OuterLoopHeader->getParent(), &errs()) &&
       "LoopInterchange handed dominance-broken IR to LCSSA rebuild");
```



https://github.com/llvm/llvm-project/pull/218468