Tidy logging code not using UNIMPLEMENTED.

Change-Id: I7a79c1671a6ff8b2040887133b3e0925ef9a3cfe
diff --git a/compiler/dex/quick/arm64/utility_arm64.cc b/compiler/dex/quick/arm64/utility_arm64.cc
index 0883694..6985b73 100644
--- a/compiler/dex/quick/arm64/utility_arm64.cc
+++ b/compiler/dex/quick/arm64/utility_arm64.cc
@@ -768,8 +768,8 @@
       opcode = kA64Sub4RRre;
       break;
     default:
-      LOG(FATAL) << "Unimplemented opcode: " << op;
-      break;
+      UNIMPLEMENTED(FATAL) << "Unimplemented opcode: " << op;
+      UNREACHABLE();
   }
   A64Opcode widened_opcode = r_dest.Is64Bit() ? WIDE(opcode) : opcode;