[Bug tree-optimization/86701] Optimize strlen called on std::string c_str()

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=86701

--- Comment #3 from Jonathan Wakely <redi at gcc dot gnu.org> ---
How can you determine that without looking at every character, at which point
you might as well just use strlen?