[Bug tree-optimization/63955] Remove pr51879-12.c xfail

[vries at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=63955

--- Comment #2 from vries at gcc dot gnu.org ---
Created attachment 34036
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=34036&action=edit
Tentative patch