Fix opmitized external array access for compound assignments

src/x64/lithium-codegen-x64.cc

Index: src/x64/lithium-codegen-x64.cc
diff --git a/src/x64/lithium-codegen-x64.cc b/src/x64/lithium-codegen-x64.cc
index 86a7e833835b56747f4c8765e893b8873465031a..ed486270ae7778777c54adb56597545710be368e 100644
--- a/src/x64/lithium-codegen-x64.cc
+++ b/src/x64/lithium-codegen-x64.cc
@@ -2311,6 +2311,7 @@ void LCodeGen::DoLoadKeyedSpecializedArrayElement(
         DeoptimizeIf(negative, instr->environment());
         break;
       case kExternalFloatArray:
+      default:
         UNREACHABLE();
         break;
     }
@@ -2964,6 +2965,7 @@ void LCodeGen::DoStoreKeyedSpecializedArrayElement(
         __ movl(Operand(external_pointer, key, times_4, 0), value);
         break;
       case kExternalFloatArray:
+      default:
         UNREACHABLE();
         break;
     }