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

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

Change-Id: Ie2f98d48f3992db568929f09737ef688fd3c139e
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>