nir, spirv_to_dxil: Add support for system-values that require a constant buffer
Since DXIL does not have support all of SPIRV's system-values, we need to add a way to lower these to values in a constant buffer.
This MR add support for nir_intrinsic_load_num_workgroups
, nir_intrinsic_load_first_vertex
, nir_intrinsic_load_is_indexed_draw
(required to distinguish between base_vertex and first_vertex), and nir_intrinsic_load_base_instance
.