Optimizing the Ion compiler back end

Wow, that dominator tree algorithm I wrote as an intern (https://bugzilla.mozilla.org/show_bug.cgi?id=666426) seems to have lasted 13 years! At the time we assumed that graphs would be small enough to not warrant the constant factor against Lengauer-Tarjan. ...And of course, browser-based apps have gotten much bigger since then.

Semi-NCA hadn't even been published yet and seems like the clear choice nowadays, so I'm glad to see it in place! Great work!

> Semi-NCA hadn't even been published yet and seems like the clear choice nowadays[.]

For those who are awkwardly lingering and casting longing glasses at the entrance door of compiler engineering like I am, and who were just as dismayed by this sentence, it wasn’t “properly” published but looks to have been described in a thesis from 2005[1] and in an extended abstract (ugh) before that[2].

But also, the reduction of RMQ to NCA, really?.. Ouch. I’m having flashbacks to my (very brief) competitive programming days, and not the good kind.

[1] https://www.cs.princeton.edu/research/techreps/TR-737-05

[2] https://www.cse.uoi.gr/~loukas/index.files/dominators_soda04...