Tidy logging code not using UNIMPLEMENTED.

Change-Id: I7a79c1671a6ff8b2040887133b3e0925ef9a3cfe
diff --git a/runtime/dex_instruction.cc b/runtime/dex_instruction.cc
index af38433..a802759 100644
--- a/runtime/dex_instruction.cc
+++ b/runtime/dex_instruction.cc
@@ -113,7 +113,7 @@
         return 1;  // NOP.
       } else {
         LOG(FATAL) << "Unreachable: " << DumpString(nullptr);
-        return 0;
+        UNREACHABLE();
       }
   }
 }