[LV] Avoid a crash when costing a uniform store which doesn't correspond to a legal scatter

If we have an unaligned uniform store, then when costing a scalable VF we can't emit code to scalarize it.  (Well, we could, but we haven't implemented that case.)  This change replaces an assert with a cost-model bailout such that we reject vectorization with the scalable VF instead of crashing.