[AArch64][SVE2] Asm: support SVE2 Floating Point Convert Group

Summary:
Patch adds support for the following intructions:

SVE2 floating-point convert precision:
    * FCVTXNT, FCVTNT, FCVTLT

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

Reviewed By: chill

Differential Revision: https://reviews.llvm.org/D62382

llvm-svn: 361801