[Bug tree-optimization/84503] [7/8 Regression] store-merging miscompilation on powerpc64 with -O3 since r241789

[jakub at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=84503

--- Comment #3 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
Created attachment 43485
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=43485&action=edit
gcc8-pr84503.patch

Untested fix.