[llvm] [VPlan] First step towards VPlan cost modeling. (PR #92555)

[Florian Hahn via llvm-commits]

https://github.com/fhahn commented:

Addressed latest comments, thanks!


> BTW, could any pre-computation be avoided by computing VPlan's cost earlier, before some VPlan transform takes place?

It would have to happen directly after initial construction, which in turn would us require to maintain a VPlan -> Cost mapping. I've left things as is for now

https://github.com/llvm/llvm-project/pull/92555