[llvm] [VPlan] Only apply forced cost to recipes with underlying values. (PR #168372)

[David Sherwood via llvm-commits]

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

LGTM, although I still think there isn't much value in trying to maintain the legacy/vplan cost model assert when the user is explicitly trying to assign arbitrary costs to things.

https://github.com/llvm/llvm-project/pull/168372