Use ArtMethod::GetImtIndex to avoid recomputing the imt index. am: cbb9de0d1b am: 97641dd7e8 am: 87f0650d39

Original change: https://android-review.googlesource.com/c/platform/art/+/2241113

Change-Id: Ie11028aae58744fb7e4f85fa107bae4518170503
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>