[llvm] [VPlan] First step towards VPlan cost modeling. (PR #67934)

[Florian Hahn via llvm-commits]

fhahn wrote:

I put up an alternative version with most of the logic moved the ::computeCost functions in VPlan, VPBasicBlock, VPRegionBlock in https://github.com/llvm/llvm-project/pull/92555

https://github.com/llvm/llvm-project/pull/67934