[AArch64][SVE] Asm: Replace 'IsVector' by 'RegKind' in AArch64AsmParser (NFC)

Patch [2/5] in a series to add assembler/disassembler support for AArch64 SVE unpredicated ADD/SUB instructions.

This change is a non functional change that adds RegKind as an alternative to 'isVector' to prepare it for newer types (SVE data vectors and predicate vectors) that will be added in next patches (where the SVE data vector is added as part of this patch set)

Patch by Sander De Smalen.

Reviewed by: rengolin

Differential Revision: https://reviews.llvm.org/D39088

llvm-svn: 317569