[Bug middle-end/38264] tree_forwarder_block_p says no to first basic block

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=38264

--- Comment #6 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
I actually just ran into a problem caused by having this check here.

I was modifying tree-ssa-ifcombine to optimize the case where we have the same
condition on both bb from a bb like:
```
int g();
int h();

int j, l;

int f(int a, int *b)
{
        if (a == 0)
        {
                if (b == &j) goto L9; else goto L7;
        }
        else
        {
                if (b == &j) goto L9; else goto L7;
        }
L7: return g();
L9: return h();
}
```

I go and try to remove one of the bb (which have the same condition) and that
should have updated dominators in a reasonable way because the removal should
have done a forwarder from bb 2 to bb3. But instead of doing the forwarding of
the bb, we end up with still bb3 and the dominator needs to be updated in
non-trival ways.