dft_tag - create tagging logic for data flow tracking

yosys> help dft_tag
dft_tag [options] [selection]
This pass... TODO
Only process $overwrite_tag and $original_tag cells.
    For each public wire that may carry tagged data, create a new public
    wire (named <wirename>:<tagname>) that carries the tag bits. Note
    that without this, tagging logic will only be emitted as required
    for uses of $get_tag.