-native-optimize 2 bug

[Matthew Fluet]

> > > When I compile test1 -native-optimize 2 and run it it appears to stick
> > > in an infinite loop.
> > 
> > conv2 of the regressions appears to exhibit similar behavior.
> 
> and harmonic and smith-normal-form.  Clearly some interaction with
> IntInf.

Just to confirm, I saw conv2 seg-fault, not enter an infinte loop;

In any event, I'm about to check in a fix that works correctly with conv2,
harmonic, and smith-normal-form.