[AArch64][SVE2] Asm: add int halving add/sub (predicated) instructions

Summary:
This patch adds support for the predicated integer halving add/sub
instructions:

    * SHADD, UHADD, SRHADD, URHADD
    * SHSUB, UHSUB, SHSUBR, UHSUBR

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

Reviewed By: rovka

Differential Revision: https://reviews.llvm.org/D62000

llvm-svn: 361136