[Bug c/81875] omp for loop optimized away

[vries at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=81875

--- Comment #4 from Tom de Vries <vries at gcc dot gnu.org> ---
Proposed to backport to 6/7 branch (
https://gcc.gnu.org/ml/gcc-patches/2017-12/msg00574.html ).