[Bug tree-optimization/95643] Optimizer fails to realize that a variable tested twice in a row is the same both times

[glisse at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95643

--- Comment #1 from Marc Glisse <glisse at gcc dot gnu.org> ---
After FRE1 we have

  _2 = x_9(D) == 0;
  if (_2 != 0)

so we assert things for _2 and not x_9, and we lose the __builtin_unreachable
information in CCP2.