Let's explicitly state the meaning of this API in its name and the code comments to make the intent clear.
This assumes that the x86 overrides of the base cost model are doing the right thing already and D43733 is improving on that. The definition and usage of this cost were not obvious in D43079 / D42981.
If clients are not using this cost as intended, then the name change should make it apparent. Out-of-tree targets will have to update for the cosmetic change, but that's an opportunity to verify that they're either ok with the base model costs or have a reasonable override for those costs.