[LV] Complete missing parts of the VPlan cost model

There are still some computeCosts functions in VPlan that haven't yet been filled in. I think that's because there were concerns about creating an accurate vplan cost model in case it triggered the (now deleted) legacy/vplan cost model assert. However, now the assert has gone we can start implementing these properly.

Locate the unimplemented computeCosts functions in VPlan and read the surrounding VPlan cost-model code first. Review the context of the deleted legacy/vplan cost model assert; done means the remaining functions implement the VPlan cost model accurately without relying on that assert.

Written by the indexing model from the issue text.