iai-callgrind now correctly exits with error if regressions were found
[1], so we no longer need to check for regressions manually. Remove this
check and instead exit based on the exit status of the benchmark run.
[1] https://github.com/iai-callgrind/iai-callgrind/issues/337
We don't actually need this for now, but eventually it would be nice to
run icount benchmarks on multiple targets. Start tagging artifact names
with the architecture, and allow passing `--tag` to `ci-util.py` in
order to retrieve the correct one.
