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

[Craig Topper via llvm-commits]

https://github.com/topperc requested changes to this pull request.

This patch isn't sound. Just because the VL is known doesn't mean the slidedown offset is <= VL. The elements that will be written to the lower elements of the destination may come from the high LMUL part of the input.

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