RFR [sh/8u]: Correct order between load, LRB and membar nodes

[Roman Kennke]

A test failed after recent LRB backport which is caused by wrong
ordering of load, LRB and trailing membar. Currently the membar feeds
off the LRB, but it should really have the raw load as its input so that
membars don't participate in the data flow analysis that we do in our
LRB optimizations.

The fix is in Unsafe accessor. This mimics how it's already done for
getfield (in parse3.cpp) and also in later releases.

Webrev:
http://cr.openjdk.java.net/~rkennke/lrb-membars/webrev.00/

Testing: the failing test doesn't crash now. hotspot_gc_shenandoah

Ok?

Roman