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

[Ashutosh Nema via llvm-commits]

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nema-ashutosh wrote:

CarriedDefToEscapingIdx maps to S.Escaping indices where 0 is a valid entry, but lookup returns 0 on a miss too — so it can't tell "absent" from "index 0". Using lookup would treat a genuine index-0 hit as a miss and create a duplicate escaping entry

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