[AArch64][SVE2] Asm: support FCVTX/FLOGB instructions

Summary:

Patch completes SVE2 support for:

    SVE Floating Point Unary Operations - Predicated Group

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

Reviewed By: SjoerdMeijer

Differential Revision: https://reviews.llvm.org/D62526

llvm-svn: 362071