This is a continuation of a fix from the last week in #108311.
The issue was not fully fixed due to a mistake in the regression
test file.
There are two major things which left to be fixed since the
previous patch:
1. Root nodes can not be shared, even if the local and distant
lights belong to the same light set. If the root node is shared
then the flattening will use the same node index for specialized
trees, which is not a desired behavior.
2. The node type needs to be preserved when a new node is
created for a subset of emitters. This is because tree sampling
in kernel will handle distant and local lights differently for
nodes where there are multiple emitters.
Pull Request: https://projects.blender.org/blender/blender/pulls/108427
5c25277f6c