Currently, build_view_index and build_instance_id emit load_instance_id
intrinsics, which want the instance ID coming into the program, which is
the true instance ID multiplied by the view count.
The loop also remaps any load_instance_id in the original program back
to the true instance ID, which is the one coming in divided by the view
count. Because we call build_view_index and build_instance_id as part
of the loop, and emit the new load_instance_id instructions earlier in
the shader, we successfully avoid seeing those.
However, this is a bit fragile as it means you can't call
build_view_index or build_instance_id prior to the loop without
accidentally remapping things we don't intend to. To fix this
fragility, we save off the original instance ID (including the view
count) and directly reference that.
Reviewed-by: Caio Oliveira <caio.oliveira@intel.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/28067>