ART: More lenient lock merging in the verifier

Be more lenient on mismatched lock levels when merging register
lines in the verifier. Registers may be aliases, in which case
it is fine to drop the lock information, as long as at least one
alias survives to keep the data alive.

Bug: 23502994
Change-Id: I0844115480286f5e6ab5ba71dced571e50dd42a5