RFR (11): Revert MachMemBarNode related diffs vs. upstream

[Roman Kennke]

Back when we had membars in our barriers (acmp I think), we needed some
extra fluff to deal with those. We can revert this in 11 now:

http://cr.openjdk.java.net/~rkennke/jdk11-revert-machmembarnode/webrev.00/

This is mostly a reversal of the upstream change (>11):

http://hg.openjdk.java.net/jdk/jdk/rev/904cf5407ac7

with some added extra cleanups in the touched files.

This cleans up our upstream diff, and reduces risks.

Testing: hotspot_gc_shenandoah

Good?

Roman