RFR (S/M): 8225478: Make G1CMRootRegions independent of HeapRegions

[Thomas Schatzl]

Hi,

 can I have reviews for this split-out from JDK-8220089 [0] from
Sangheon? In internal discussions we decided to split up that CR into
multiple parts, but since he's on vacation now I want to get this
reviewed as I want to base some more cleanup work/split-outs on this
one.

Basically, for JDK-8220089 and other purposes we want refactoring for
the G1CMRootRegions class to hold MemRegions instead of HeapRegions.
G1CMRootRegions implicitly uses ntams/top, but in JDK-8220089 top()
(for survivor regions) will not be constant any more at mutator time.
So we need to take a real snapshot of the memory areas that are roots.
That will also help for other future changes.

Since this is basically a split-out of that functionality of JDK-
8220089 webrev.4 [1] (review thread [2]) apart from some minor comment
editing (i.e. review work), Sangheon will be stated the author.

Which means, that I will act as reviewer, with this change "looks
good"ed :)

CR:
https://bugs.openjdk.java.net/browse/JDK-8225478
Webrev:
http://cr.openjdk.java.net/~tschatzl/8225478/webrev/
Testing:
hs-tier1-3 (Sangheon already pushed 8220080 through hs-tier1-8).


[0] https://bugs.openjdk.java.net/browse/JDK-8220089
[1] http://cr.openjdk.java.net/~sangheki/8220089/webrev.4/
[2] 
http://mail.openjdk.java.net/pipermail/hotspot-gc-dev/2019-May/025774.html