[RISCV] Refactor lowerSPLAT_VECTOR_PARTS to use splatPartsI64WithVL for scalable vectors.

There was quite a bit of duplication between splatPartsI64WithVL
and the scalable vector handling in lowerSPLAT_VECTOR_PARTS, but
scalable vector had one additional case. Move that case to
splatPartsI64WithVL which improves some fixed vector tests.

Reviewed By: reames

Differential Revision: https://reviews.llvm.org/D158876