Optimizing: Fix codegens for MethodLoadKind::kDexCacheViaMethod.

Use the original method index instead of the target method
index because the target method may point to a different dex
file.

No regression test: this currently happens only if the
codegen uses the kDexCacheViaMethod as a fallback for
another load kind and we aim to avoid that fallback, so it
would be difficult to write a reliable regression test. We
could try and exploit current fallbacks for irreducible
loops on x86 and arm but those fallbacks will eventually
disappear anyway.

Bug: 28036230
Change-Id: I4cc9e046480d3d60a7fb521f0ca6a98914625cdc