ProcArrayGroupClearXid() compare-exchange style

ProcArrayGroupClearXid() has this:

    while (true)
    {
        nextidx = pg_atomic_read_u32(&procglobal->procArrayGroupFirst);

        ...

        if (pg_atomic_compare_exchange_u32(&procglobal->procArrayGroupFirst,
                                           &nextidx,
                                           (uint32) proc->pgprocno))
            break;
    }

This, from UnpinBuffer(), is our more-typical style:

        old_buf_state = pg_atomic_read_u32(&buf->state);
        for (;;)
        {
            ...

            if (pg_atomic_compare_exchange_u32(&buf->state, &old_buf_state,
                                               buf_state))
                break;
        }

That is, we typically put the pg_atomic_read_u32() outside the loop.  After
the first iteration, it is redundant with the side effect of
pg_atomic_compare_exchange_u32().  I haven't checked whether this materially
improves performance, but, for style, I would like to change it in HEAD.