AArch64InstPrinter: rewrite of printSysAlias

This is a cleanup/rewrite of the printSysAlias function. This was not using the
tablegen instruction descriptions, but was "manually" decoding the
instructions. This has been replaced with calls to lookup_XYZ_ByEncoding
tablegen calls.

This revealed several problems. First, instruction IVAU had the wrong encoding.
This was cancelled out by the parser that incorrectly matched the wrong
encoding. Second, instruction CVAP was missing from the SystemOperands tablegen
descriptions, so this has been added. And third, the required target features
were not captured in the tablegen descriptions, so support for this has also
been added.

Differential Revision: https://reviews.llvm.org/D30329

llvm-svn: 296343