ART: Ignore try blocks with no throwing instructions

In order to avoid complex removal of redundant exceptional edges in
the SSA builder, this patch modified the graph builder to consider
blocks without throwing instructions as not in a try block, even if
covered by a TryItem.

In some corner cases, this may generate more TryBoundaries than
necessary, but those can be removed once the SSA form is built.

Change-Id: I158c4542b2c1964a8dd532f82e921b9cb1997e1e