Goes atop !459. The last six commits belong to this MR.
-- v5: vkd3d-shader/dxil: Implement the DXIL LOAD instruction. vkd3d-shader/dxil: Implement the DXIL GEP instruction. vkd3d-shader/dxil: Support global variable initialisers. vkd3d-shader/dxil: Introduce a value type for immediate constant buffers. vkd3d-shader/dxil: Implement default address space global variables. vkd3d-shader: Delete unused struct list from struct vkd3d_shader_indexable_temp.