[codegen] Rename and explicit usage of replace_with_alias;
[codegen] Rename and explicit usage of replace_with_alias;

It can actually only replace one result; don't try to make it generic
yet, since there's no point in doing so right now, and make it do the
dumb thing so it's not surprising to use.

stoklund/cretonne,stoklund/cretonne,stoklund/cretonne