[Bug tree-optimization/58689] Enhance returns_nonnull

[glisse at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=58689

--- Comment #1 from Marc Glisse <glisse at gcc dot gnu.org> ---
5) A warning that suggests adding the attribute to a function, when gcc can
detect that the return value is != 0.