[MLton] bug in refFlatten

[Matthew Fluet]

> There appears to be a subtle bug in the refFlatten pass.  The pass seems
> to be sensitive to the ordering of functions in the SSA2 program
> representation.  While it would be o.k. (though, less than ideal) to get
> different results depending on the order of functions, in this situation, 
> the semantics of the resulting program changes.
> 
> The bug is exhibitted by ref-flatten.sml from the regression suite.  To 
> trigger the bug, write a simple SSA2 that simply reverses the order of 
> functions in the program representation.  Run this pass immediately before 
> the refFlatten pass.  The program prints 13 instead of 44.

Alternatively, just add
	val functions = List.rev functions
at the top of the refFlatten pass, though a separate reverse pass is a 
little easier to turn on/off from the command line.