[llvm] [VPlan] Sink retrieving legacy costs to more specific computeCost impls. (PR #109708)

[via llvm-commits]

https://github.com/ayalz approved this pull request.

Thanks for reworking this, LGTM!
Sinking legacy costs into individual usages will hopefully help refactor them into VPlan. 
Added some minor comments.

https://github.com/llvm/llvm-project/pull/109708