ART: cleanup exit_block_ in graph if exit block is removed

If we remove the exit block from the graph (for example method
contains infinite loop) we should also clean the field exit_block_
in graph. At least inliner expects it.

Change-Id: Icda668da2233cdd6cd673635a1949f5ed34cf270
Signed-off-by: Serguei Katkov <serguei.i.katkov@intel.com>