[llvm] [LV][EVL] Introduce MergeUntilPivot VPInstruction to enable out-loop reduction in EVL vectorization. (PR #101641)

[Mel Chen via llvm-commits]

Mel-Chen wrote:

@fhahn Change to emit VPWidenIntrinsicRecipe. If you think this appoarch is better, please let me know. I will remove unused legacy code for VPInstruction::MergeUntilPivot later.

https://github.com/llvm/llvm-project/pull/101641