[Bug c++/79639] [6/7 Regression] ICE with -O and constexpr

[jakub at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=79639

--- Comment #2 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
Created attachment 40805
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=40805&action=edit
gcc7-pr79639.patch

Untested fix.  Not really sure about it though.