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

[Luke Lau via llvm-commits]

lukel97 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.

I should specify that this is stacked upon #65997, which is in turn stacked upon #65916. Will add this to the PR description

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