When compiling BSDF nodes, we only assing stack space to the normal and
tangent inputs if they are linked. However, it could be that the
ConstantFolder removed the link, so checking if there is a link fails
to take this into account.

To fix this, added a flag to ShaderInput to keep track of whether a
constant was folded into the input, and use it as well to verify that
the socket is linked when assigning stack space.