[RISCV][NFC] Add policy operand for RISCVISD::VSLIDEUP_VL and RISCVISD::VSLIDEDOWN_VL.

There is room for optimization to use tail agnostic vslideup/vslidedown to lower
some vector operations. D125546 is an revision for the kind of optimization.

Reviewed By: craig.topper

Differential Revision: https://reviews.llvm.org/D140393