[AArch64][SVE] Add SVE2 intrinsics for pairwise arithmetic

Summary:
Implements the following intrinsics:
 - addp
 - smaxp, sminp, umaxp & uminp
 - sadalp & uadalp

Reviewers: dancgr, efriedma, sdesmalen, c-rhodes

Reviewed By: c-rhodes

Subscribers: tschuett, kristof.beyls, hiraditya, rkruppe, psnobl, cameron.mcinally, cfe-commits, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D73347