More store/allocation elimination.

When merging values of an unreturned singleton's location,
do more analysis on the singleton's liveness since the singleton
may not be live in some of the predecessors. And only those
predecessors where the singleton is live should participate in
merging the value of the location.

Test: make test-art-host
Bug: 31716107

Change-Id: I1bf61a84d3473646eb38fd9924bbb063cf664f7f