[llvm] [RISCV] Combine build_vector of extract_vector_elts to vector_shuffle (PR #80883)

[Philip Reames via llvm-commits]

preames wrote:

Did you explore the impact of adjusting SDAG construction to pad the mask with undefs in this case?  That would seem like a more reasonable canonical form that then explode and build.  We could maybe have it behind a hook to preserve existing target behavior if needed.

https://github.com/llvm/llvm-project/pull/80883