Clean up `ObjectReference<>` use with vreg values.

Clean up the object reference costructors and move
the `AsVRegValue()` to the `CompressedReference<>`.
Add `CompressedReference::FromVRegValue()` that can
be used to convert vreg values back to references.

Test: testrunner.py --host
Test: m test-art-host-gtest
Bug: 172332525
Change-Id: Id60618c0922b51591d0180bbe93e7ffabb598f56
4 files changed