Tidy logging code not using UNIMPLEMENTED.

Change-Id: I7a79c1671a6ff8b2040887133b3e0925ef9a3cfe
diff --git a/compiler/compiler.cc b/compiler/compiler.cc
index fbfd8e6..36213ca 100644
--- a/compiler/compiler.cc
+++ b/compiler/compiler.cc
@@ -75,8 +75,8 @@
 
     default:
       LOG(FATAL) << "UNREACHABLE";
+      UNREACHABLE();
   }
-  return nullptr;
 }
 
 }  // namespace art