[AArch64][SVE2] Asm: support SVE2 Histogram Computation Groups

Summary:
Patch adds support for the following instructions:

SVE2 histogram generation (segment):
    * HISTSEG

SVE2 histogram generation (vector):
    * HISTCNT

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

Reviewed By: chill

Differential Revision: https://reviews.llvm.org/D62306

llvm-svn: 361796