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.