Shading: Refactor Math node and use dynamic inputs.

- Implement dynamic inputs. The second input is now unavailable in single
operand math operators.
- Reimplemenet the clamp option using graph expansion for Cycles.
- Clean up code and unify naming between Blender and Cycles.
- Remove unused code.

Reviewers: brecht

Differential Revision: https://developer.blender.org/D5481

intern/cycles/render/nodes.h

@@ -1260,11 +1260,12 @@ class MathNode : public ShaderNode {
   {
     return NODE_GROUP_LEVEL_1;
   }
+  void expand(ShaderGraph *graph);
   void constant_fold(const ConstantFolder &folder);
 
   float value1;
   float value2;
-  NodeMath type;
+  NodeMathType type;
   bool use_clamp;
 };