Return statements never flow to the next block (or any other block, for
that matter), so drawing arrows leaving them not only messes up dot's
graphs, but is quite missleading.
The output can be controlled via --block-dot (not yet documented). The
files a named <sourcefile>.<function>.<stage>.dot. Currently, stage will be
one of "initial" (after expression to statement conversion), "thread"
(after jump threading), "dead" (after dead block removal), "final" (final
state before actual code emission).
