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

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

Change-Id: I0fecdd864662725626bc122f504707c94d9c1a68
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>