Dump the graph before/after every pass

We will now dump a partial CFG when encountering an error while
compiling which is useful for debugging a crash.

Also, force the number of threads to 1 if the --dump-cfg option is
passed. This makes the visualizer_dump_mutex_ obsolete since there's no
possibility of a race.

Bug: 203755436
Test: ART tests, dumped cfg with an added CHECK(false) to verify the
partial .cfg

Change-Id: I55bc9ceafef9e9eb4a43e891fb88d10e619bf64c