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.