[llvm] [RISCV] Pack build_vectors into largest available element type (PR #97351)

[Philip Reames via llvm-commits]

preames wrote:

Quick note for reviewers, please pay close attention to byte order issues in this.  I frequently get that wrong, and while this is inspired by a downstream patch I rewrote this basically from scratch.  Definitely room for error.  

https://github.com/llvm/llvm-project/pull/97351