Tidy logging code not using UNIMPLEMENTED.

Change-Id: I7a79c1671a6ff8b2040887133b3e0925ef9a3cfe
diff --git a/compiler/utils/arm/assembler_thumb2.cc b/compiler/utils/arm/assembler_thumb2.cc
index 7968a77..37478c4 100644
--- a/compiler/utils/arm/assembler_thumb2.cc
+++ b/compiler/utils/arm/assembler_thumb2.cc
@@ -2406,6 +2406,7 @@
       break;
     default:
       LOG(FATAL) << "UNREACHABLE";
+      UNREACHABLE();
   }
 }
 
@@ -2477,6 +2478,7 @@
       break;
     default:
       LOG(FATAL) << "UNREACHABLE";
+      UNREACHABLE();
   }
 }