[lworld] Move null check from method entry into i2c adapter

[Tobias Hartmann]

Hi,

please review the following change that moves the value type argument null check from method entry
to the i2c adapter:
http://cr.openjdk.java.net/~thartmann/valhalla/lworld/adapter_null_check/

Like this, we only check for null when calling a compiled method from the interpreter. Calls from
compiled code don't need a check because we can never pass null there (we catch null at checkcasts
and value type returns). The hack in sharedRuntime.cpp is currently required because the adapter
sharing code does not have enough bits in the fingerprint to represent T_VALUETYPEPTR (we can fix
that later with the calling convention changes).

Thanks,
Tobias