Only allow instance-of to improve knowledge of a type for downcasts.

Previous "not upcast" test could improve a type to an impossible type that
following a merge back with the original register would lead to conflict
(and subsequent verifier errors).
Modify UpdateRegisters so that the work line will be updated if merging
causes changes in the fall-through case.

Bug: 15808277
Issue: https://code.google.com/p/android/issues/detail?id=72093
Change-Id: Ib16cae8506246177e902825af036d5a397ad0dac