[VE] VE Vector Predicated SDNode, vector add isel and tests

VE Vector Predicated (VVP) SDNodes form an intermediate layer between VE
vector instructions and the initial SDNodes.

We introduce 'vvp_add' with isel and tests as the first of these VVP
nodes. VVP nodes have a mask and explicit vector length operand, which
we will make proper use of later.

Reviewed By: kaz7

Differential Revision: https://reviews.llvm.org/D91802