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

[Philip Reames via llvm-commits]

preames wrote:

@topperc Have I addressed all of your concerns?  This has an LGTM, but you pointed out a substantial problem after that, so I don't want to land without confirmation.  

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