[AArch64][AsmParser] Extend RegOp with integrated 'shift/extend'.

Summary:
In some cases the shift/extend needs to be explicitly parsed together
with the register, rather than as a separate operand. This is needed
for addressing modes where the instruction as a whole dictates the
scaling/extend, rather than specific bits in the instruction.
By parsing them as a single operand, we avoid the need to pass an
extra operand in all CodeGen patterns (because all operands need to
have an associated value), and we avoid the need to update TableGen to
accept operands that have no associated bits in the instruction.

An added benefit of parsing them together is that the assembler
can give a sensible diagnostic if the scaling is not correct.

This is patch [2/4] 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

Reviewed By: fhahn, SjoerdMeijer

Subscribers: kristof.beyls, llvm-commits

Differential Revision: https://reviews.llvm.org/D45688

llvm-svn: 330394