[Bug tree-optimization/111388] std:.get_if variant, unnecessary branch when outside of if statement

[pinskia at gcc dot gnu.org via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=111388

--- Comment #2 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Adding:
```
        if (ab.index()>=2)
          __builtin_unreachable();
```
to operator->

Also fixes the issue.
C++23 would be:
[[assume(ab.index()<2)]];
(Except that does not optimize currently).