Use ArtMethod::GetImtIndex to avoid recomputing the imt index. am: cbb9de0d1b am: 97641dd7e8 am: 87f0650d39 am: a010b3ad83

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

Change-Id: Iec859161e28939c9f6be33b9532331eba3c06d70
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>