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

Only apply forced instruction costs to recipes with underlying values to
match the legacy cost model. A VPlan may have a number of additional
VPInstructions without underlying values that are not considered for its
cost, and assigning forced costs to them would incorrectly inflate its
cost.

This fixes a cost divergence between legacy and VPlan-based cost models
with forced instruction costs.

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

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