[RISCV] Lower scalar_to_vector

Loads of fixed length vectors with irregular element counts are
sometimes emitted as a scalar load + scalar_to_vector.
Previously the scalar_to_vector wasn't legal and so was scalarized
further. This patch handles it by lowering it to a vmv.s.x.

Reviewed By: craig.topper

Differential Revision: https://reviews.llvm.org/D147608