[RISCV] Pad v4i1/v2i1/v1i1 stores with 0s to make a full byte.

As noted in the FIXME there's a sort of agreement that the any
extra bits stored will be 0.

The generated code is pretty terrible. I was really hoping we
could use a tail undisturbed trick, but tail undisturbed no
longer applies to masked destinations in the current draft
spec.

Fingers crossed that it isn't common to do this. I doubt IR
from clang or the vectorizer would ever create this kind of store.

Reviewed By: frasercrmck

Differential Revision: https://reviews.llvm.org/D100618