Unreproducible bug

[Matthew Fluet]

I've started getting some strange compiler errors:

x_0 has no num occurrences
or
f_0 is not in scope

I think they're related property lists, but I can't think of anything that
I've changed that could effect them.  And, these are occuring using the
SML/NJ version of MLton, not a self-compiled version.  I've gotten it a
couple of times on the regression suite, but when I rerun it on the same
file, there's no error.  In the last 4 days, I haven't modified anything
outside of the x86-codegen directory, with the exception of adding a
traceBatch function to control.sml and adding a flag to main.sml.  Has
anyone every seen this before?