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

[Ramkumar Ramachandra via llvm-commits]

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artagnon wrote:

```suggestion
  return VMap.lookup_or(V, V);
```

Could introduce ValueMap::lookup_or similar to DenseMap::lookup_or?

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