u128xN casts missing

[TheIronBorn]

Any particular reason for this?

(It speeds up a component of rand's UniformInt by up to x2. We could implement it with the platform intrinsic but that's not ideal)

[gnzlbg]

Its probably just an oversight, the _128xN types are new.

[gnzlbg]

Which casts are missing here? They do look correct to me.