VertexInterpolators allocate unused params which generate extra shader ops

As a follow up on this: https://udn.unrealengine.com/s/question/0D54z00006yxUdXCAU/vertexinterpolators-problem

VertexInterpolators are added to the HLSL code even if unused and you demand the HLSL compiler to strip away unused functions (not a big deal if that code compiles). Another issue I noticed is that the unused vertexinterpolator functions access some scalar/vector params and view params that are kept in the allocated buffers (even if the function is removed by the compiler).

In some of our shaders I noticed an increase in total op count just by adding an unused (and stripped) Time * Param vertex interpolator.

I attached two pixelshaders:

58_ps.txt: comes from a simple material (basecolor+normal+cavity*specular+roughnes+ao)
60_ps.txt comes from the same material to with I added an unconnected Time * Speed param VertexInterpolator