Avoid messing up loop notes 3 take 2

[Jan Hubicka]

> On Fri, Jul 27, 2001 at 06:48:26PM +0200, Jan Hubicka wrote:
> > 	* expr.c (loop_not_expected): New global variable.
> > 	* rtl.h (loop_not_expected): Declare.
> > 	* flow.c (try_forward_edges): Avoid removing of loop pre-headers
> > 	before loop.
> > 	* toplev.c (rest_of_compilation): Set loop_not_expected.
> 
> I sorta liked the CLEANUP_PRE_LOOP version better.
OK, I've considered it somewhat fragile, as someone later may come,
call cfg_cleanup once before loop optimizer without the flag and blast
it off.

I will update the old version and resend then.
Once we get indepdendent on loop notes, we should probably blast off
the pre-headers always and then re-create them for each natural loop
found in the loop optimizer.

This is little bit tricky with LOOP notes as they may lurk around on
inexpected places.

Honza