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

Bug: http://b/111067277
Test: builds
Change-Id: I8b69ea3815e14bb6eb27f40c0dd01a85b340a355
diff --git a/compiler/optimizing/code_generator_x86.cc b/compiler/optimizing/code_generator_x86.cc
index fba4da6..1b74d22 100644
--- a/compiler/optimizing/code_generator_x86.cc
+++ b/compiler/optimizing/code_generator_x86.cc
@@ -1228,7 +1228,7 @@
     case DataType::Type::kUint64:
     case DataType::Type::kVoid:
       LOG(FATAL) << "Unexpected parameter type " << type;
-      break;
+      UNREACHABLE();
   }
   return Location::NoLocation();
 }
@@ -2989,7 +2989,7 @@
 
     default:
       LOG(FATAL) << "Unexpected add type " << add->GetResultType();
-      break;
+      UNREACHABLE();
   }
 }