[MLton] property list cleanups

[Stephen Weeks]

> This graph doesn't get too big?  It should be "bigger" than the
> complete CFG for the program, since it has edges for every variable
> in every Goto.

True, but it's not any bigger than the syntax tree for the program,
since the program already has to have space for every variable in
every goto.

I didn't see a significant effect on the running time of the pass of
commonArg during a self compile.  It was ~3s before and still is.
Same with Joe's huge program.