[MLton] Int Overflow detection

[Matthew Fluet]

> I wanted to compare mlton's performance with gcc on this platform (cygwin, x86)
> and wanted a fair test.  

The test is fair in the sense that it computes the same value, but unfair
in the sense that this is not a typical SML program, nor a program that is
susceptible to the kinds of optimizations towards which MLton has invested 
time and effort.

> I am only showing the main loop here (this is with int overflow turned off). 
> So it's 4 instructions for gcc versus 9 for mlton.

I doubt this has any significant affect on run-time performance.

> I have not timed the loops.  If there is interest I could do that.

I think that would be more instructive than counting instructions.