RFR: Cleanup code in C2 related to G1/Sh pre-barrier handling

[Roland Westrelin]

> http://cr.openjdk.java.net/~rkennke/cleanup-c2-pre-barrier/webrev.00/

There are things in there that should not be removed.

The pre barriers for stores to non escaping allocation can be
eliminated. To locate the pre barrier the call node has an extra edge to
the address of the store. If the store is eliminated for some reason
then we don't need to keep the address computation alive. That's what:

CallLeafNode::Ideal()
the code in Compile::remove_useless_nodes()
the code in kill_dead_code()
the code in PhaseIterGVN::remove_globally_dead_node()
the code in Node::set_req_X()

take care of. For kill_dead_code() and
PhaseIterGVN::remove_globally_dead_node(), enqueue_useful_gc_barrier()
could cover that.

Also the code in macro.cpp is required for scalarization to work.

Roland.