[Bug c++/82781] [6/7/8 Regression] Vector extension operators return wrong result in constexpr

[jakub at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=82781

--- Comment #3 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
Created attachment 42624
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=42624&action=edit
gcc8-pr82781.patch

Untested fix.