System Information
Win10, 8GB RAM, GTX 570

Blender Version
2.78c e92f235283

Short description of error
Related to 2c58e96685e8.
Cycles skips unnecessary mix (1 or 0 for Fac) and multiply node inputs of constant value (~1s render time: good):

However, when Fac or one of the multiply inputs is a variable, both inputs are calculated regardless if it should be skipped (~5s render time for identical result: bad):

In other words, despite the input value being 0 and the output being completely black, the render take the same time as it would with all the noise visible (~5s)

Noise stress-test (~5s):

Material was rendered on 10x10 plane at 50% 1080p, however it can drastically scale in performance decrease. Adding/fixing this optimization could drastically reduce render times depending on the complexity of a shader.