System Information
windows, gtx980

Blender Version
2.75
Worked: (optional)
2.74

Short description of error
When a node with float input (like a math node) is connected to a color, the color was converted using a weighted average based on human perception:

f = dot(col, vec3(0.35, 0.45, 0.2)) until 2.74,

Now, it's

(col.r, col.g, col.b) / 3.0

no matter if a vector or color is connected to the float input.

Exact steps for others to reproduce the error

Attached blend file: export a shader in 2.74 and 2.75, (gpu.export_shader) and diff them
In 2.74, the add node function is clalled by sth like

math_add(dot(tmp2.rgb, vec3(0.35, 0.45, 0.2)), dot(tmp4.rgb, vec3(0.35, 0.45, 0.2)), tmp7);

in 2.75, it's

math_add((tmp2.r + tmp2.g + tmp2.b) / 3.0, (tmp4.r + tmp4.g + tmp4.b) / 3.0, tmp7);