[llvm] [RISCV] Reduce LMUL when index is known when lowering insert_vector_elt (PR #66087)

[Philip Reames via llvm-commits]

preames wrote:

It looks like maybe you pushed the wrong set of commits to this PR?  The first couple of changes seem unrelated to your proposed commit message.

https://github.com/llvm/llvm-project/pull/66087