[Bug tree-optimization/94801] Failure to optimize narrowed __builtin_clz

[glisse at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94801

--- Comment #2 from Marc Glisse <glisse at gcc dot gnu.org> ---
  if(a==0)__builtin_unreachable();

lets gcc optimize the code.