DCE eliminating valid statement for ACATS c34007p

[Richard Kenner]

    The only useful definition is that valid GENERIC is what the
    gimplifier can turn into valid GIMPLE, which is much more well-defined
    ;) Modulo bugs in the gimplifier of course ...

But that's the whole problem!  If you have a tree that the gimplifier
can't correctly process, how do you determine whether it's not valid
GENERIC or whether the gimplifier has a bug?  Using a tautology as the
definition isn't helpful in that process.