ART: Update graph's exit block field if removed

Running DCE on an infinite loop will delete the exit block but the
corresponding field is currently not cleared in the parent graph.
This does not cause any problems at the moment as that information is
only used in codegens to DCHECK that a block is not the exit block.
However, it will be necessary to update the inliner once we start to
inline methods with loops.

With this patch, DCE will update the HGraph::exit_block_ field. DCHECK
was also added to HGraph::InlineInto to make sure that the inlined
graph does have an exit block.

Change-Id: Ia8ddca375bbc6830cd919af6059a52cc9b73a023