Given a big Varnode, find the placeholder corresponding to the logical value given by a size and significance offset. If it doesn't exist, create it. \param vn is the big Varnode containing the logical value \param bitSize is the size of the logical value in bytes \param lsbOffset is the signficance offset of the logical value within the Varnode \return the found/created placeholder
source not stored for this graph (policy: none)