Clean up `ObjectReference<>` use with vreg values. am: 970291ce79 am: 853a5f3f4d

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

Change-Id: I56ab804184bab89db400666077d5f35015349241
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>