Blender Git Loki

Cycles: constant fold add/mul type nodes with known 0 and 1 arguments.

These values often either turn the node into a no-op, or even make it
evaluate to 0 no matter what the other input value is, thus allowing
deletion of a branch of the node graph that otherwise is not constant.

Reviewed By: brecht

Differential Revision: https://developer.blender.org/D2085