Clean up `ObjectReference<>` use with vreg values. am: 970291ce79

Original change: https://android-review.googlesource.com/c/platform/art/+/2388217

Change-Id: I927166572b67201cc5e75353daf7d6a4ba0d15fe
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>