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

[Ramkumar Ramachandra via llvm-commits]

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artagnon wrote:

```suggestion
Value *LoopSplitUtils::getPartitionValue(Value *V,
                                         unsigned PartitionIndex) const {
  assert(PartitionIndex < getNumPartitions() && "partition index out of range");
  // Partition 0 reuses the original loop: every value maps to itself.
  if (PartitionIndex == 0)
    return V;
```

Would be good to avoid the const_cast?

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