[M68k] Convert register Aliases to AltNames

This makes it simpler to determine when two registers are actually the
same vs just partially aliasing.

The only real caveat is that it becomes impossible to know which name
was used for the register previously. (i.e. parsing assembly and then
disassembling it can result in the register name changing.)

Differential Revision: https://reviews.llvm.org/D98536