ART: Fix issues with SplitBlock

The SplitBlock is a public interface from MIRGraph to be able to create two
blocks from one. This interface can be called not just during the control
flow graph building phase, but later. At that later point, extended instructions
may have already been inserted.

For example, SpecialMethodInliner can insert kMirOpNullCheck and kMirOpNop,
and then another pass tries to split block that contains this. The DCHECK
will fail even though the split is semantically correct.

Signed-off-by: Mathew Zaleski <mathew.zaleski@intel.com>

Conflicts:

	compiler/dex/mir_graph.cc

Change-Id: I8bb3551407907909cda444a3379da2eb9331b9ff