[AArch64][AsmParser] Add isScalarReg() and repurpose isReg()

Summary:
isReg() in AArch64AsmParser.cpp is a bit of a misnomer, and would be better named 'isScalarReg()' instead.

Patch [1/3] in a series to add operand constraint checks for SVE's predicated ADD/SUB.

Reviewers: rengolin, mcrosier, evandro, fhahn, echristo

Reviewed By: fhahn

Subscribers: aemerson, javed.absar, llvm-commits, kristof.beyls

Differential Revision: https://reviews.llvm.org/D41445

llvm-svn: 321646