Cycles: Refactor of constant fold.

* Move constant folding from nodes to the shader graph. This way it's part of our (later) 4-step optimization process.
* Instead of only doing a one level constant fold, we can now do a recursive constant fold, allowing us to simplify shaders much further.
Constant folding is implemented for Blackbody, Math and VectorMath nodes.

Example (the highlighted nodes are removed before rendering):
Before: http://archive.dingto.org/2015/blender/code/one_level_constant_fold.jpg
Now: http://archive.dingto.org/2015/blender/code/multi_level_constant_fold.jpg

Thanks to Sergey and Brecht for Review!
Differential Revision: https://developer.blender.org/D1626

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