v3d/compiler: implement nir_intrinsic_load_base_instance

v3d/compiler: implement nir_intrinsic_load_base_instance

Vulkan lowers gl_InstanceIndex to load_base_instance +
load_instance_id, so we need to implement loading the base instance in
the compiler.

The base instance is set by the BASE_VERTEX_BASE_INSTANCE command
right before the instanced draw call and it is included in the VPM
payload together with the InstanceID and VertexID if this is requested
by the shader record.

Reviewed-by: Alejandro Piñeiro <apinheiro@igalia.com>
Reviewed-by: Eric Anholt <eric@anholt.net>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/6766>