[RISCV] Simplify scalable vector case in lowerVectorMaskExt.

Since we have SPLAT_VECTOR_PARTS these days, I don't think we need
to go through extra lengths to avoid introducing an illegal scalar type.
We can just call getConstant using the scalable vector type and let
it create either a SPLAT_VECTOR or a SPLAT_VECTOR_PARTS.

Reviewed By: frasercrmck, rogfer01

Differential Revision: https://reviews.llvm.org/D121645