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

[Craig Topper via llvm-commits]

topperc wrote:

This patch isn't sound. Just because the VL is known doesn't mean the slidedown offset is <= VL.

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