RFR: 8287215: Add method to scan bitmap backwards [v4]

[Thomas Schatzl]

> Hi all,
> 
>   can I have reviews for this change that adds a method to scan bitmaps backwards? This functionality is necessary for upcoming change to remove on of the mark bitmaps [JDK-8210708](https://bugs.openjdk.java.net/browse/JDK-8210708).
> 
> As such this is only an addition of that functionality with no reference (apart from the gtest) from Hotspot itself.
> 
> Testing: gtest
> 
> Thanks,
>   Thomas

Thomas Schatzl has updated the pull request incrementally with one additional commit since the last revision:

  Make r_index exclusive

-------------

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/8869/files
  - new: https://git.openjdk.java.net/jdk/pull/8869/files/721d2eb4..6382a48a

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=8869&range=03
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=8869&range=02-03

  Stats: 13 lines in 3 files changed: 5 ins; 1 del; 7 mod
  Patch: https://git.openjdk.java.net/jdk/pull/8869.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/8869/head:pull/8869

PR: https://git.openjdk.java.net/jdk/pull/8869