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

[Michael Maitland via llvm-commits]

michaelmaitland wrote:

> getSmallestVTForIndex won't reduce LMUL to anything lower than M1, so this patch shouldn't go from M1/MF2 or MF4.

This is good enough for me.

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