constant switch test bug in x86Translate

[Matthew Fluet]

> > 1. if the test is an immediate, choose the right label.
> >    pros: a simple fix
> >    cons: why do it in the codegen when the cps simplifier normally does it
> 
> I vote for this solution.  I'm guessing it's a small amount of SML code, and it

O.k., it is easy enough.  I think I'll integrate this solution with the
3rd solution as well; I've never seen it happen in practice, but there's
no reason why the Transfer.replace bug couldn't be raised by the
copyPropagation phase under normal circumstances.