how to widen vector in target defined intrinsics

Hello Developers,
I add an Intrinsic in my target called int_npp_veltadd using llvm-11.0.0,
Its declaration is <64 x i8> @llvm.nnp.veltadd(<64 x i8> %a, <64 x i8> %b, <64 x i8> %c, <64 x i8> %d).

But when the input is <28 x i8>, my backend get an error in LegalizeVectorTypes.cpp line 2733, it says “Do not know how to widen the result of this operator!”. Because of the vector <28 x i8> is extended, the function TLI.getOperationAction return EXPAND. So the target’s ReplaceNodeResults cannot be called. so I cannot widen the vector to 64 x i8 in my target.
how can I widen vector in this situation?
thanks a lot!~