[AArch64][SVE2] Asm: support WHILE instructions

Summary:
Patch adds support for the following instructions:
    * WHILEGE, WHILEGT, WHILEHS, WHILEHI, WHILEWR, WHILERW

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

Reviewed By: chill

Differential Revision: https://reviews.llvm.org/D62601

llvm-svn: 362215