vkd3d-shader/ir: Introduce a better CFG structurizer, part 1.

This first MR create an internal structured representation of the CFG and computes the domination relationship. Over a few other MRs I will implement an algorithm inspired to this article to compute a better structure than the one we already have, but for the moment the computed data is immediately wasted. A working prototype of the new structurizer is now available in https://gitlab.winehq.org/giomasce/vkd3d/-/commits/cfg4, though further improvement is planned on top of that too.