[RISCV] Use getSplatBuildVector instead of getSplatVector for fixed vectors.

The splat_vector will be legalized to build_vector eventually
anyway. This patch makes it take fewer steps.

Unfortunately, this results in some codegen changes. It looks
like it comes down to how the nodes were ordered in the topological
sort for isel. Because the build_vector is created earlier we end up
with a different ordering of nodes.

Reviewed By: frasercrmck

Differential Revision: https://reviews.llvm.org/D122185