Add the RegisterCompleter to eArgTypeRegisterName in g_argument_table (#82428)

This is a follow-on to:

https://github.com/llvm/llvm-project/pull/82085

The completer for register names was missing from the argument table. I
somehow missed that the only register completer test was x86_64, so that
test broke.

I added the completer in to the right slot in the argument table, and
added a small completions test that just uses the alias register names.
If we end up having a platform that doesn't define register names, we'll
have to skip this test there, but it should add a sniff test for
register completion that will run most everywhere.