[Bug tree-optimization/92131] incorrect assumption that (ao >= 0) is always false

[arigo at tunes dot org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=92131

--- Comment #7 from Armin Rigo <arigo at tunes dot org> ---
Created attachment 47056
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=47056&action=edit
made the example runnable

Here is a main().  Compare:
* gcc -Og foomin3.c foomin3main.c && a.out
* gcc -O1 foomin3.c foomin3main.c && a.out