[llvm] [Transforms][Utils] Add LoopSplitUtils for iteration-space loop splitting (PR #205995)

[Ramkumar Ramachandra via llvm-commits]

================

----------------
artagnon wrote:

```suggestion
    auto Existing = CarriedDefToEscapingIdx.lookup(LiveOutDef);
    auto &EV = Existing
                   ? S.Escaping[Existing]
                   : S.addEscaping(LiveOutDef);
```

https://github.com/llvm/llvm-project/pull/205995