[AArch64][SVE] Add SVE2 intrinsic for xar

Summary: Implements the @llvm.aarch64.sve.xar intrinsic

Reviewers: andwar, c-rhodes, dancgr, efriedma, rengolin

Reviewed By: andwar

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

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D75160