This replaces `stack_assign` with `stack_assign_if_linked`, which should save a few SVM nodes for constant parameters.
Running benchmarks (all scenes in the benchmark repo, 3 runs, median value for each) shows 1.0% improvement on CPU and 1.5% on OptiX. Not huge, but fairly (all between -0.2% and 3.0%).
Pull Request: https://projects.blender.org/blender/blender/pulls/143404
3107d1f962