ART: Change merge(uninitialized, null) to produce conflict

The verifier would treat the result of merge(uninitialized, null) as
an uninitialized value, allowing a constructor to be called on null.
This patch conservatively prevents any usage of the merged register
by treating it as conflicting. Note that this could be relaxed with
a new common type for initialized and uninitialized references.

Bug: 26579108
Bug: 22411633

Change-Id: Ic5a1a3909c80287deece92dbb3254f437074e9b6