[CostModel][X86] Improve InsertElement costs for sub-128bit vectors

If we're inserting into v2i8/v4i8/v8i8/v2i16/v4i16 style sub-128bit vectors ensure we don't use the SK_PermuteTwoSrc cost of the legalized value type - this is a followup to rG12c629ec which added equivalent sub-128bit shuffle costs