RFR: 8287215: Add method to scan bitmap backwards

[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

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Commit messages:
 - 8287215: Add method to scan bitmap backwards

Changes: https://git.openjdk.java.net/jdk/pull/8869/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=8869&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8287215
  Stats: 241 lines in 4 files changed: 229 ins; 0 del; 12 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