Blender Git Loki

Cycles: Experiment with use of runtime tag instead of set for SVM generation

This gives about 2x speedup (3.2sec vs. 11.9sec with 32716 handled nodes) when
updating shader for the shader tree.

Reviewers: brecht, juicyfruit, dingto, lukasstockner97

Differential Revision: https://developer.blender.org/D1700