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
diff --git a/compiler/optimizing/code_generator_arm.cc b/compiler/optimizing/code_generator_arm.cc
index 98577d6..f45aa66 100644
--- a/compiler/optimizing/code_generator_arm.cc
+++ b/compiler/optimizing/code_generator_arm.cc
@@ -6440,8 +6440,9 @@
                         reg,
                         method_reg,
                         ArtMethod::DexCacheResolvedMethodsOffset(kArmPointerSize).Int32Value());
-      // temp = temp[index_in_cache]
-      uint32_t index_in_cache = invoke->GetTargetMethod().dex_method_index;
+      // temp = temp[index_in_cache];
+      // Note: Don't use invoke->GetTargetMethod() as it may point to a different dex file.
+      uint32_t index_in_cache = invoke->GetDexMethodIndex();
       __ LoadFromOffset(kLoadWord, reg, reg, CodeGenerator::GetCachePointerOffset(index_in_cache));
       break;
     }