[llvm] [Transforms][Utils] Add LoopSplitUtils, a reusable iteration-space loop splitter (PR #209142)

[Ramkumar Ramachandra via llvm-commits]

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artagnon wrote:

Would be good to remove this, and default to true, so we can commit to the migration, and have it tested fully?

https://github.com/llvm/llvm-project/pull/209142