[AArch64][SVE2] Asm: add saturating add/sub instructions

Summary:
Patch adds support for the following instructions:

    * SQADD, UQADD, SUQADD, USQADD
    * SQSUB, UQSUB, SQSUBR, UQSUBR

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

Reviewed By: chill

Differential Revision: https://reviews.llvm.org/D62130

llvm-svn: 361611