intel: Handle firstvertex in an identical way to BaseVertex

intel: Handle firstvertex in an identical way to BaseVertex

Until we set gl_BaseVertex to zero for non-indexed draw calls
both have an identical value.

The Vertex Elements are kept like that:
* VE 1: <BaseVertex/firstvertex, BaseInstance, VertexID, InstanceID>
* VE 2: <Draw ID, 0, 0, 0>

v2 (idr): Mark nir_intrinsic_load_first_vertex as "unreachable" in
emit_system_values_block and fs_visitor::nir_emit_vs_intrinsic.