[AArch64][SVE2] Asm: support SVE2 Narrowing Group

Summary:
Patch adds support for the following instructions:

SVE2 bitwise shift right narrow:
    * SQSHRUNB, SQSHRUNT, SQRSHRUNB, SQRSHRUNT, SHRNB, SHRNT, RSHRNB, RSHRNT,
      SQSHRNB, SQSHRNT, SQRSHRNB, SQRSHRNT, UQSHRNB, UQSHRNT, UQRSHRNB,
      UQRSHRNT

SVE2 integer add/subtract narrow high part:
    * ADDHNB, ADDHNT, RADDHNB, RADDHNT, SUBHNB, SUBHNT, RSUBHNB, RSUBHNT

SVE2 saturating extract narrow:
    * SQXTNB, SQXTNT, UQXTNB, UQXTNT, SQXTUNB, SQXTUNT

The specification can be found here:
https://developer.arm.com/docs/ddi0602/latest

Reviewed By: SjoerdMeijer

Differential Revision: https://reviews.llvm.org/D62205

llvm-svn: 361624