[Bug sanitizer/70342] g++ -fsanitize=undefined never finishes compiling (>24h) in qtxmlpatterns test suite

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=70342

--- Comment #4 from Richard Biener <rguenth at gcc dot gnu.org> ---
1011    cp_fold_function (tree fndecl)
1012    {
1013      cp_walk_tree (&DECL_SAVED_TREE (fndecl), cp_fold_r, NULL, NULL);
1014    }

so we walk duplicates more than once.  Changing it to use
cp_walk_tree_without_duplicate fixes this.

So it seems -fsanitize=undefined introduces some tree cycles in what
cp_walk_tree
walks (for example via sharing tree nodes).