[llvm] [LV] Return Invalid from getLegacyCost when instruction cost forced. (PR #154543)

[Kerry McLaughlin via llvm-commits]

kmclaughlin-arm wrote:

This PR fixes another issue I found whilst testing #147535 with LNT.
When passing `-force-target-instruction-cost=1`, oggenc fails to build as it hits the "VPlan cost model and legacy cost model disagreed" assert.

https://github.com/llvm/llvm-project/pull/154543