ART: Fix simplification of catch blocks in the presence of dead code

Simplification of catch blocks transforms the code so that catch
blocks have only exceptional predecessors. However, it is invoked
before trivially dead code is eliminated which breaks simple
assumptions such as the fact that a catch block cannot start with
move-exception if it has non-exceptional predecessors. This patch
fixes the algorithm to work under these relaxed conditions.

Bug: 25494450
Bug: 25492628
Change-Id: Idc8d010102a4b8b9a6cd918b98d6e11d1838db0c