[llvm] [VPlan] Remove legacy costing inside VPBlendRecipe::computeCost (PR #171846)

[David Sherwood via llvm-commits]

https://github.com/david-arm approved this pull request.

LGTM! Seems like a sensible change, and good to see more legacy cost model being removed from VPlan!

https://github.com/llvm/llvm-project/pull/171846