[AArch64AsmParser] rewrite of function parseSysAlias

This is a cleanup/rewrite of the parseSysAlias function. It was not using the
tablegen instruction descriptions, but was “manually” matching the mnemonics
and recreating the operands whereas all this information is already in
tablegen; all this code has been replaced with calls to lookupXYZByName
tablegen calls.

Differential Revision: https://reviews.llvm.org/D30491

llvm-svn: 296857