For use on the exception path.  Rather than emitting the instructions
for a bunch of decrefs, instead just emit a single call to xdecrefAll()

Improves performance quite a lot: sre_parse_parse llvm instructions goes
from 360k to 80k, and it's 60k if I manually disable cxx fixups.
Overall time [which is entirely compile time] goes from 12.5s to 4.4s,
though master does it in 1.3s so there's still some work to do.  But
even if I turn off cxx fixups entirely it still takes 2.9s.