[llvm] [VPlan] Add initial VPScalarEvolution, use to get trip count SCEV (NFC) (PR #94464)

[Renato Golin via llvm-commits]

rengolin wrote:

This looks ok to me, but not yet clear how the new `VPScalarEvolution` class will be used inside `VPlanAnalysis`. 

Will it just be a helper class? If so, why not a static method? If not, then how will it be composed across different VPlans and the analysis part? I don't have a strong opinion here, just trying to understand the direction.

https://github.com/llvm/llvm-project/pull/94464