Implement Reference.refersTo() intrinsic.

Test: Added tests to 122-npe and 160-read-barrier-stress
Test: m test-art-host-gtest
Test: testrunner.py --host --optimizing
Test: run-gtests.sh
Test: testrunner.py --target --optimizing
Bug: 172573708
Change-Id: I8342510565289058df218d3249ffac1eb993ca4f
15 files changed