This goes atop !598 and !603.
-- v8: vkd3d-shader/ir: Introduce a simple control flow graph structurizer. vkd3d-shader/ir: Handle PHI nodes when materializing SSA registers. vkd3d-shader/ir: Materialize SSA registers to temporaries. vkd3d-shader/spirv: Support bool TEMP registers. vkd3d-shader/spirv: Move bool casting helpers above register loading helpers. vkd3d-shader/spirv: Convert the swizzle according to the source bit width. vkd3d-shader/dxil: Set the register before calling src_param_init_scalar().