[RISCV][VP] Custom lower VP_SCATTER and VP_GATHER

This patch adds support for the `VP_SCATTER` and `VP_GATHER` nodes by
lowering them to RVV's `vsox`/`vlux` instructions, respectively. This
process is almost identical to the existing `MSCATTER`/`MGATHER` support.

One extra change was made to `SelectionDAGLegalize` so that
`VP_SCATTER`'s operation action is derived from its stored "value"
operand rather than its return type (which is always the chain).

Reviewed By: craig.topper, rogfer01

Differential Revision: https://reviews.llvm.org/D108987