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

[Andrei Elovikov via llvm-commits]

eas wrote:

> How about something like CurrentTripCount. 

This can be confusing as well, one might thing about number of iterations remaining (maybe?). How about `[Orig|Scalar]IterationsProcessed`?

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