[Bug tree-optimization/53265] Warn when undefined behavior implies smaller iteration count

[rguenth at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=53265

--- Comment #6 from Richard Biener <rguenth at gcc dot gnu.org> 2013-03-11 14:16:20 UTC ---
To warn,
1) add a flag to struct loop whether we warned for the loop already
2) in both discover_iteration_bound_by_body_walk and
maybe_lower_iteration_bound
   warn if you run into an estimate we end up using and that lowers the
   max iterations below the value returned by number_of_latch_executions ().