ART: Fix NullCheckElimination, BBCombine, and SplitBlock

ART: Fix NullCheckElimination, BBCombine, and SplitBlock

NullCheckElimination had one issue and one assumption that could be
broken:
-It ignored that compiler temps may hold references.
-Assumed there are no phi nodes even though algorithm can be run even
after phi nodes are inserted.

BBCombine also had issue in that it did not properly maintain the
instruction links. The logic has been updated to use utility methods.

SplitBlock has an issue with being called after extended instructions
are inserted. Namely, in the case in question, it was called after
SpecialMethodInliner was through and although it was doing semantically
correct thing, it was hitting dcheck due to the kMirOpNullCheck.

Change-Id: Id5863ddb0762064e74bf1d9173b8db5cb47cf3b9
Signed-off-by: Razvan A Lupusoru <razvan.a.lupusoru@intel.com>