[Bug c/48228] incorrect signed overflow warning when only 0 and 1 are used

[rguenth at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=48228

--- Comment #3 from Richard Guenther <rguenth at gcc dot gnu.org> 2011-03-22 
10:46:49 UTC ---
Which happens because of our way of handling iteration with PHIs.  Which
of course is very overly conservative with single-arg PHIs (or PHIs w/o
backedges).