[MLton] Catching redundant pattern matches

[Matthew Fluet]

> Quoting Jesper Louis Andersen (jlouis@mongers.org):
> >
> > I will try updating to recent sources and then check again.
> >
>
> found, I had "deadCode true" in my sources in an ann.

That is _very_ dangerous to use outside the Basis Library implementation.
We consider it an undocumented "feature", so if you were led to include it
in your sources for any reason other than cut-n-paste-ing from the BL
implementation, please let us know.