RFR: vbox/vunbox should throw ClassCastException if target type is invalid

[Tobias Hartmann]

Hi,

please review the following patch:
http://cr.openjdk.java.net/~thartmann/valhalla/vt_prototype/webrev.19/

The runtime/valhalla/valuetypes/VboxUnbox.java test currently fails with a guarantee during parsing because the target type of vbox/vunbox is invalid (see VboxUnbox::testIncorrectBoxing()t's 'String'). C2 should throw a ClassCastException in this case.

Thanks,
Tobias