Blender Git Loki

Cycles: add folding for redundant A to B to A conversions.

As a result of other folding simplifications it may happen that
two type conversion nodes end up directly connected. In some
cases it may be possible to then remove both. A realistic case
might be an optimized out Mix RGB node used to blend vectors.

It seems it's safe to optimize when B is a float3 type
(color, vector), and A is float3 or float.

Reviewers: #cycles, sergey

Reviewed By: #cycles, sergey

Subscribers: sergey

Differential Revision: https://developer.blender.org/D2134