RFR: Fix C2 intrinsics barriers

[Roland Westrelin]

> - two other instances seem to require explicit barriers even though
> make_unsafe_addr() should already insert them. Possible that the
> verifier is too strict here. Re-inserted those explicit barriers with
> comment. We should check what's up there

make_unsafe_addr() only adds barriers for cases where we can't say
whether the access is on heap or not. Otherwise access_load/access_store
take care of the barriers. For these 2 intrinsics, there's only a
runtime call.

Roland.