Goes atop !459. The last five commits belong to this MR.
-- v4: 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.