[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 #2 from Armin Rigo <arigo at tunes dot org> ---
Created attachment 47054
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=47054&action=edit
slightly different version, with comments showing the expected values