RFR(S): 8189613: [MVT] __Value oop not preserved when safepointing at return

[Tobias Hartmann]

Hi,

please review the following patch:
https://bugs.openjdk.java.net/browse/JDK-8189613
http://cr.openjdk.java.net/~thartmann/8189613/webrev.00/

When safepointing at a __Value return, the code in ThreadSafepointState::handle_polling_page_exception() does not 
preserve the oop because ValueKlass::returned_value_type(map) returns NULL. This leads to all kinds of crashes. We need 
to set 'return_oop' to true whenever we return a value type that cannot be passed in registers.

The webrev also includes some refactoring.

All tests pass.

Thanks,
Tobias