C++17 compatibility: make WITH_TIDY=1 happy again.

Bug: http://b/111067277
Test: builds
Change-Id: I8b69ea3815e14bb6eb27f40c0dd01a85b340a355
diff --git a/compiler/compiled_method.cc b/compiler/compiled_method.cc
index ef9d919..58f7e4f 100644
--- a/compiler/compiled_method.cc
+++ b/compiler/compiled_method.cc
@@ -73,7 +73,7 @@
     }
     default:
       LOG(FATAL) << "Unknown InstructionSet: " << instruction_set;
-      return 0;
+      UNREACHABLE();
   }
 }
 
@@ -94,7 +94,7 @@
     }
     default:
       LOG(FATAL) << "Unknown InstructionSet: " << instruction_set;
-      return nullptr;
+      UNREACHABLE();
   }
 }