ART: Changes to try-catch in GraphBuilder

This patch adds an additional case into the insertion algorithm for
HTryBoundary inside HGraphBuilder in order to better handle catch
blocks covered by a TryItem.

Building SSA form also required to stop combining HTryBoundaries for
neighbouring TryItems because it was not clear which exception
handlers belong to which try block.

Change-Id: Ic68bd6ef98fee784609fa593cb08dca1f00a15e0