gas: fold do_repeat{,_with_expander}()

do_repeat_with_expander() already deals with the "no expander" case
quite fine, so there's really little point having two functions. What it
lacks compared with do_repeat() is a call to sb_build(), which can
simply be moved (and the then redundant sb_new() be avoided). Along with
this moving also flip if the main if()'s condition such that the "no
expander" case is handled first.