Scheduler takes too much time to order control flow nodes

In optimized builds the initialization in Hlt2Conf.test_hlt2_all_lines takes about 500 seconds. In debug builds, it almost never finishes within the 1 hour limit.

This also affects the build of the functor cache (it adds about one hour to the total build time).

This is a consequence of the new (un)packing configuration (!3268 (merged))

I'm preparing a change that optimizes this part.