TableGen: use correct MIOperand when printing aliases

Previously, TableGen assumed that every aliased operand consumed precisely 1
MachineInstr slot (this was reasonable because until a couple of days ago,
nothing more complicated was eligible for printing).

This allows a couple more ARM64 aliases to print so we can remove the special
code.

On the X86 side, I've gone for explicit AT&T size specifiers as the default, so
turned off a few of the aliases that would have just started printing.