[Bug middle-end/87290] Optimize signed x % pow2p == cst

[jakub at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=87290

--- Comment #2 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
Created attachment 44687
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=44687&action=edit
gcc9-pr87290.patch

Untested patch.