+ /// Join the congruence classes of two registers. This function is biased
+ /// towards the left argument, i.e. after
+ ///
+ /// addReg(r2);
+ /// unionRegs(r1, r2);
+ ///
+ /// the leader of the unioned congruence class is the same as the leader of
+ /// r1's congruence class prior to the union. This is actually relied upon
+ /// in the copy insertion code.