[AArch64][SVE2] Add intrinsics for binary narrowing operations

Summary:
The following intrinsics for binary narrowing add and sub operations are
added:
  * @llvm.aarch64.sve.addhnb
  * @llvm.aarch64.sve.addhnt
  * @llvm.aarch64.sve.raddhnb
  * @llvm.aarch64.sve.raddhnt
  * @llvm.aarch64.sve.subhnb
  * @llvm.aarch64.sve.subhnt
  * @llvm.aarch64.sve.rsubhnb
  * @llvm.aarch64.sve.rsubhnt

Reviewers: sdesmalen, rengolin, efriedma

Reviewed By: sdesmalen, efriedma

Subscribers: tschuett, kristof.beyls, hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D71424