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/test/render_graph_finalize_test.cpp

@@ -1003,7 +1003,7 @@ TEST_F(RenderGraph, constant_fold_math_clamp)
  * Includes 2 tests: constant on each side.
  */
 static void build_math_partial_test_graph(ShaderGraphBuilder &builder,
-                                          NodeMath type,
+                                          NodeMathType type,
                                           float constval)
 {
   builder