[VPlan] Only use isAddressSCEVForCost in legacy getAddressAccSCEV (NFCI)
Follow-up to https://github.com/llvm/llvm-project/pull/171204 and
1f331e453f to only rely on isAddressSCEVForCost in legacy isAddressSCEVForCost,
completely aligning the decisions of VPlan and legacy cost model.