[RISCV] Prefer vmv.s.x for build_vector a, undef, ..., undef#136164

preames merged 2 commits intollvm:mainllvm/llvm-project:mainfrom

preames:pr-riscv-splat-via-vmv.s.xpreames/llvm-project:pr-riscv-splat-via-vmv.s.xCopy head branch name to clipboard