[Bug c++/70847] [6/7 Regression] exponential time in cp_fold for chained virtual function calls

[jakub at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=70847

--- Comment #13 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
Created attachment 38638
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=38638&action=edit
gcc7-pr70847.patch

So, shall I test this version?
As added bonus, if cp_fold returns for different stmts the same return value
for whatever reason, we wouldn't walk the subtrees on it more than once.