Loosen the assertion a bit to make the test suite pass with -O.

Running the test quite with check-swift-optimize revealed that metatypes were not covered by the assert, even though it may occur in the real code.

Swift SVN r28297

lib/SIL/DynamicCasts.cpp

@@ -239,8 +239,9 @@ swift::classifyDynamicCast(Module *M,
               target.getStructOrBoundGenericStruct() ||
               isa<TupleType>(target) ||
               isa<SILFunctionType>(target) ||
-              isa<FunctionType>(target)) &&
-             "Target should be an enum, struct, tuple or function type");
+              isa<FunctionType>(target) ||
+              isa<MetatypeType>(target)) &&
+             "Target should be an enum, struct, tuple, metatype or function type");
       return DynamicCastFeasibility::WillFail;
     }