Shenandoah: C2: Make in-native LRB special case of normal LRB

Currently, IN_NATIVE LRB generates a runtime call during parsing in C2. This may break optimizations. It is known to break the mirror->Klass*->mirror / Klass*->mirror->Klass* optimizations, and possibly some others too (e.g. wholesale elimination of the barrier on new/constant objects).

It seems better to emit the normal LoadReferenceBarrier, with a special property 'native' instead, and only generate different runtime-call-addresses.