[AArch64][AsmParser] NFC: Cleanup parsing of scalar registers.

Summary:
- Renamed tryParseRegister to tryParseScalarRegister, which
  now returns an OperandMatchResultTy.
- Moved matching of certain aliases into matchRegisterNameAlias.
- Changed type of most 'Reg' variables to 'unsigned'.

This is patch [1/4] in a series to add assembler/disassembler support for
SVE's contiguous LD1 (scalar+scalar) instructions:
- Patch [1/4]: https://reviews.llvm.org/D45687
- Patch [2/4]: https://reviews.llvm.org/D45688
- Patch [3/4]: https://reviews.llvm.org/D45689
- Patch [4/4]: https://reviews.llvm.org/D45690

Reviewers: fhahn, rengolin, javed.absar, huntergr, SjoerdMeijer, t.p.northover, echristo, evandro, samparker

Reviewed By: samparker

Subscribers: samparker, llvm-commits, kristof.beyls

Differential Revision: https://reviews.llvm.org/D45687

llvm-svn: 330311