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.
I think it would be better remove the op0 fields from the other three alias classes, as it is defined to always be 0b01 for all of them.