vkd3d-shader/ir: Transform clip/cull inputs into an array.

D3D12 supports up to eight clip/cull values, which are naturally best implemented as an array, but DXBC/TPF lacks array support, so it declares up to two vectors of four. SPIR-V uses an array. Instead of converting the vectors in the signature into an I/O array element spanning up to eight register indices, with all the complexity that would entail, we use dedicated clip/cull registers. Addressing is fixed up in the normaliser, and the backend need only compute the array size(s) and declare the dedicated register(s).