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

Summary:
The following intrinsics for binary narrowing shift righ operations are
added:
  * @llvm.aarch64.sve.shrnb
  * @llvm.aarch64.sve.uqshrnb
  * @llvm.aarch64.sve.sqshrnb
  * @llvm.aarch64.sve.sqshrunb
  * @llvm.aarch64.sve.uqrshrnb
  * @llvm.aarch64.sve.sqrshrnb
  * @llvm.aarch64.sve.sqrshrunb
  * @llvm.aarch64.sve.shrnt
  * @llvm.aarch64.sve.uqshrnt
  * @llvm.aarch64.sve.sqshrnt
  * @llvm.aarch64.sve.sqshrunt
  * @llvm.aarch64.sve.uqrshrnt
  * @llvm.aarch64.sve.sqrshrnt
  * @llvm.aarch64.sve.sqrshrunt

Reviewers: sdesmalen, rengolin, efriedma

Reviewed By: efriedma

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

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D71552