[llvm] [RISCV] Combine vslidedown_vl with known VL and offset to a smaller LMUL (PR #66267)

[Luke Lau via llvm-commits]

lukel97 wrote:

Marking as a draft as it needs rebased, and may not be worthwhile doing if #66671 is not able to fully subsume the lowering logic for insert_vector_elt and insert_subvector

https://github.com/llvm/llvm-project/pull/66267