Generalise ASSIGN-TN-DEPTHS to expose the reduce function

By default, this still computes the maximum loop depth at which a
TN is accessed, but, e.g., changing that to #'+ will compute the
sum of the depths at which the TN appears.

Also, process all the references to each TN at a time, instead of
walking down the list of IR2 blocks to update every TN that appear
in each block.