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).