[MLton] bug with packed representations

[Stephen Weeks]

> Unfortunately, projecting the low bits via a Cast doesn't mesh well with
> the x86 codegen's translation of a Cast:
> 
> 	  | Cast (z, _) => toX86Operand z
> 
> And I think this is the correct interpretation.  Thus far, we've only used
> Cast to indicate that we were changing our interpretation of bits, but not
> the actual bits themselves (or the number of bits).
...
> It would seem to me that we should require a Cast statement to preserve
> the number of bits;

Absolutely.  I'll add this to the Rssa and Machine type checkers.
Thanks for the find.