[MLton] moving refFlatten before commonSubexp

[Stephen Weeks]

Once we port all our Ssa optimizations over to Ssa2, one thing that we
will be able to do is move refFlatten before commonSubexp.  This could
help refFlatten, because commonSubexp may decrease the lifetime of an
object that contains a ref cell.  This could make it impossible for
refFlatten to prove (because it may not be true) that the container is
live at a point of use of the ref cell, and hence disable flattening
of that cell.