c/7344: performance regression on huge case statements

[Nathanael Nerode]

http://gcc.gnu.org/cgi-bin/gnatsweb.pl?cmd=view%20audit-trail&database=gcc&pr=7344

To clarify what I meant:
 >The call to note_prediction_to_br_prob was added to 
rest_of_compilation >in toplev.c with the change from revision 1.620 to 
1.621 of toplev.c.
 >Sure enough, everything is fine with the May 7th CVS.
In the May 9th CVS,
 >CC1 spends a large amount of time cycling through basic blocks, but it 
 >doesn't hang yet. I haven't pinned down the patch which causes it to 
 >hang.

So there's a slowdown when the call is introduced, but it isn't enough 
to cause the infinite execution time.  A subsequent change between May 
9th and July 17th must have done that, and I haven't pinned down what
change that is.