regex and overload: unifiy 1 and N arg branches

regex and overload: unifiy 1 and N arg branches

When compiling a regex, something like /a$b/ that parses two two args,
was treated in a different code path than /$a/ say, which is only one arg.

In particular the 1-arg code path, where it handled "" overloading, didn't
check for a loop (where the ""-sub returns the overloaded object itself) -
the N-arg branch did handle that. By unififying the branches, we get that
fix for free, and ensure that any future fixes don't have to be applied to
two separate branches.

Re-indented has been left to the commit that follows this.