Tidy logging code not using UNIMPLEMENTED.

Change-Id: I7a79c1671a6ff8b2040887133b3e0925ef9a3cfe
diff --git a/runtime/gc/heap.cc b/runtime/gc/heap.cc
index bceac44..c0008aa 100644
--- a/runtime/gc/heap.cc
+++ b/runtime/gc/heap.cc
@@ -1325,8 +1325,9 @@
               // Throw OOM by default.
               break;
             default: {
-              LOG(FATAL) << "Unimplemented homogeneous space compaction result "
-                         << static_cast<size_t>(result);
+              UNIMPLEMENTED(FATAL) << "homogeneous space compaction result: "
+                  << static_cast<size_t>(result);
+              UNREACHABLE();
             }
           }
           // Always print that we ran homogeneous space compation since this can cause jank.
@@ -1761,7 +1762,8 @@
         break;
       }
       default: {
-        LOG(FATAL) << "Unimplemented";
+        UNIMPLEMENTED(FATAL);
+        UNREACHABLE();
       }
     }
     if (IsGcConcurrent()) {