Eevee: implement conditional evaluation of Mix node branches.

If the material effectively combines multiple distinct materials
using some kind of mask texture, it is wasteful to evaluate all
of them when the mask fully excludes some. Cycles already supports
this optimization for Mix Shader nodes.

This implements a similar feature for Mix Shader and Mix Color Blend
nodes in Eevee: shader matches Cycles, and mixing colors can be used
for a similar purpose in NPR shaders.

To achieve that, a Conditional node type directly supported by code
generation is added. Shader nodes can add these conditionals as needed,
and the code generator partitions the node graph into a branch tree
and appropriately generates conditionals. Empty conditionals are
automatically eliminated to avoid any performance impact. This
processing is done separately for every sub-graph to minimize
dependency cross-contamination.

Differential Revision: https://developer.blender.org/D16218

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