This PR adds a bit more granularity to the output of e.g. `clif-util
compile -T`, indicating how much time is spent in VCode lowering and
various other new-backend-specific tasks.
that only one value ever flows. Has been observed to improve generated code
run times by up to 8%. Compilation cost increases by about 0.6%, but up to 7%
total cost has been observed to be saved; iow it can be a significant win in
terms of compilation time, overall.
