[AArch64][AsmParser] Cleanup isSImm7s4, isSImm7s8, (etc) functions.

Reviewers: fhahn, rengolin, t.p.northover, echristo, olista01, samparker

Reviewed By: fhahn, samparker

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

Differential Revision: https://reviews.llvm.org/D41899

llvm-svn: 322481