[RISCV] Convert VSLIDE(UP|DOWN) nodes to "VL" versions (NFC)

This patch prepares the RISCV VSLIDEUP and VSLIDEDOWN custom nodes to
ones carrying additional mask and vector-length operands. This is
primarily so they can be used by both systems.

This also takes the opportunity to create some helper functions to deal
with the common task of getting the default (unmasked) VL operands.

Reviewed By: craig.topper, arcbbb

Differential Revision: https://reviews.llvm.org/D96505