Blender Git Loki

Cycles: Implement node deduplication routines

The idea of this commit is to merge nodes which has identical settings
and matching inputs into a single node in order to minimize number of
SVM instructions.

This is quite simple bottom-top graph traversal and the trickiest part
is how to compare node settings without too much trouble which seems to
be solved is quite clean way.

Still possibilities for further improvements:

- Support comparison of BSDF nodes
- Support comparison of volume nodes
- Support comparison of curve mapping/ramp nodes

Reviewers: brecht, juicyfruit, dingto

Differential Revision: https://developer.blender.org/D1673