[llvm] [VPlan] Introduce VPNumProcessedElementsPHIRecipe for variable-length stepping (PR #177114)

[Shih-Po Hung via llvm-commits]

arcbbb wrote:

> It would be good if you could add a bit more detail on why this is needed and what the benefit is vs the EVL based IV. The description mostly describes how things to moved around, but it would be good to clarify why this is needed.

Thanks! I've updated the description to clarify the motivation. Please take another look!

https://github.com/llvm/llvm-project/pull/177114