[VP] ISD helper functions [VE] isel for vp_add, vp_and

This implements vp_add, vp_and for the VE target by lowering them to the
VVP_* layer. We also add helper functions for VP SDNodes (isVPSDNode,
getVPMaskIdx, getVPExplicitVectorLengthIdx).

Reviewed By: kaz7

Differential Revision: https://reviews.llvm.org/D93766