vkd3d-shader/ir: Lower monolithic switches to selection ladders.

This is a preliminary pass for the control flow graph structurizer. The plan, at least for the first version of the structurizer, is to have this pass, then another pass that converts all SSA registers to TEMPs (and correspondingly PHI nodes to MOVs/MOVCs), then a simple structurizer based on the so-called Böhm–Jacopini theorem. Some changes to the SPIR-V backend will also be needed, for supporting BOOL TEMP registers and fixing some details of 64 bit types.

But this pass is already quite complicated, so large enough for its own MR.