vkd3d-shader/ir: Introduce a better CFG structurizer, part 6.

This applies on top of !704 (merged), the last two commits belong here.

Here we finally emit the structured program, replacing the older simple structurizer. The advantage of the new structurizer is that the CFG if statically described in the generated program, without having to trace the value of a variable. Upcoming MRs will make the CFG even simpler, easier to read and hopefully to optimize for downstream compilers.