RFR: enqueue barrier + some other things

[Roland Westrelin]

> One question: the EQ barrier node replaces the incoming value. But it
> doesn't itself produce any new value (like RB/WB do), does it?

Once expanded, it doesn't produce a value.

> - Sure you need the 'class MemoryFixer' there?
> http://cr.openjdk.java.net/~roland/shenandoah/enqueue-barrier/webrev.00/src/hotspot/share/opto/loopnode.hpp.udiff.html

Indeed, I'll remove it.

Roland.