[RISCV] Don't reduce vslidedown's VL in rotations

Even though we only need to write to the bottom NumElts - Rotation
elements for the vslidedown.vi, we can save an extra vsetivli toggle if
we just keep the wide VL.

(I may be missing something here: is there a reason why we want to explicitly keep the vslidedown narrow?)

Reviewed By: craig.topper

Differential Revision: https://reviews.llvm.org/D151390