RFR: 8304016: Add BitMap find_last suite of functions

[Kim Barrett]

Please review this change that adds functions to BitMap for finding the last
set/clear bit in a range.

Testing:
mach5 tier1, including new gtesting for the new functions.

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Commit messages:
 - find_last_set/clear_bit

Changes: https://git.openjdk.org/jdk/pull/12988/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=12988&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8304016
  Stats: 216 lines in 3 files changed: 160 ins; 24 del; 32 mod
  Patch: https://git.openjdk.org/jdk/pull/12988.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/12988/head:pull/12988

PR: https://git.openjdk.org/jdk/pull/12988