New codegen segfault tickled by your patch

[Edward Z. Yang]

Hey Simon,

I was investigating the newly introduced segfault from the new
codegenerator, and found that if I obliterated this patch, the
problem went away:

    pruneSparkQueue: handle CAFs

I'd appreciate it if you could stare at the patch some, and see
if it is buggy; if not, some info on what it does would greatly
aid my debugging process. :-)

Cheers,
Edward