RFR(S): 8211993: [lworld] TestNativeClone fails with assert(!obj->is_ValueType()) failed: should not cast value type

[Tobias Hartmann]

Hi,

please review the following patch:
https://bugs.openjdk.java.net/browse/JDK-8211993
http://cr.openjdk.java.net/~thartmann/8211993/webrev.00/

C2 tries to emit a virtual call to Object.clone() on a value type receiver and fails when trying to
cast a ValueTypeNode in VirtualCallGenerator::generate(). Instead of fixing all the call sites of
null_check_receiver_before_call() and cast_not_null() we should just skip null checking/casting for
value types.

Thanks,
Tobias