"wonder IRs in JSC" Can you please provide a link/explanation to what JSC is? Did you mean this? https://github.com/eatonphil/jsc
DFG ThreadedCPS and DFG SSA are not well documented. That might be a monumental task at this point. They started simple but got so insane. But here are some slides: http://www.filpizlo.com/slides/pizlo-splash2018-jsc-compiler... http://www.filpizlo.com/slides/pizlo-speculation-in-jsc-slid...
And no, the thing you linked to is not t he JSC I think of.