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

I'm tired of investigating this, but it seems likely that the problem
was introduced with the introduction of et-forest.c, since that's where 
the loop is.  This was introduced by Pavel Nejedly and committed to 
mainline by Jan Hubicka (along with most of the surrounding code).

(So that's why I'm ccing you; it looks like you caused it, so maybe you 
  can figure out how to fix it. :-/)

--Nathanael