RFR (M): 8071278: Fix the closure mess in G1RemSet::refine_card()

[Thomas Schatzl]

Hi all,

  one line:

"188 lines changed: 23 ins; 143 del; 22 mod; 3097 unchg"

This is a rip-out and manual collapsing of needlessly complex code in
the update rs algorithm.

There are more improvements obvious now in that area (JDK-8071280, JDK-
8162928, JDK-8175554 at least), so I kept this one to the minimum.

The change makes the code a bit faster too due to avoiding repeated
null-checks, region-crossing checks etc. etc.

CR:
https://bugs.openjdk.java.net/browse/JDK-8071278
Webrev:
http://cr.openjdk.java.net/~tschatzl/8071278/webrev/
Testing:
jprt, tier2+3 testing, some benchmarks

Thanks,
  Thomas