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

[Ramkumar Ramachandra via llvm-commits]

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artagnon wrote:

```suggestion
      for (Use &U : EV.Def->uses())
        if (auto *User = dyn_cast<Instruction>(U.getUser()))
          if (!L->contains(User))
            Updater.RewriteUse(U);
```

If there is some iterator-invalidation issue, could use make_early_inc_range or to_vector?

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