Add intrinsics for unary narrowing operations

Summary:
The following intrinsics for unary narrowing operations are added:
 * @llvm.aarch64.sve.sqxtnb
 * @llvm.aarch64.sve.uqxtnb
 * @llvm.aarch64.sve.sqxtunb
 * @llvm.aarch64.sve.sqxtnt
 * @llvm.aarch64.sve.uqxtnt
 * @llvm.aarch64.sve.sqxtunt

Reviewers: sdesmalen, rengolin, efriedma

Reviewed By: efriedma

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

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D71270