arglist: optimize flush_pre_post(), and __iadd__() with it

Unless an argument is marked as Dedup.OVERRIDDEN, pre_flush_set and
post_flush_set will always be empty and the loops in flush_pre_post()
will not be doing anything interesting:

        for a in self.pre:
            dedup = self._can_dedup(a)
            if a not in pre_flush_set:
                # This just makes new a copy of self.pre
                new.append(a)
                if dedup is Dedup.OVERRIDDEN:
                    # this never happens
                    pre_flush_set.add(a)

        for a in reversed(self.post):
            dedup = self._can_dedup(a)
            if a not in post_flush_set:
                # Here self.post is reversed twice
                post_flush.appendleft(a)
                if dedup is Dedup.OVERRIDDEN:
                    # this never happens
                    post_flush_set.add(a)
        new.extend(post_flush)

In this case it's possible to avoid expensive calls and loops, instead
relying as much on Python builtins as possible.  Track whether any options
have that flag and if not just concatenate pre, _container and post.

Before:

   ncalls  tottime  cumtime
    45127    0.251    4.530 arglist.py:142(__iter__)
    81866    3.623    5.013 arglist.py:108(flush_pre_post)
    76618    3.793    5.338 arglist.py:273(__iadd__)

After:

    35647    0.156    0.627 arglist.py:160(__iter__)
    78998    2.627    3.603 arglist.py:116(flush_pre_post)
    73774    3.605    5.049 arglist.py:292(__iadd__)

The time in __iadd__ is reduced because it calls __iter__, which flushes
pre and post.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

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