Overall this is simpler, results in less verbose SPIR-V code which is easier to debug, and eliminates stuttering issues in Horizon Zero Dawn when run in an SM 6 dev branch with Gio's cfg4 structuriser branch.
-- v4: vkd3d-shader/ir: Materialise SSAs to temps before lowering switch instructions. vkd3d-shader/ir: Convert SSAs to temps only if the block of origin does not dominate all uses.