RFR: 8304016: Add BitMap find_last suite of functions [v2]

[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.

Kim Barrett has updated the pull request incrementally with two additional commits since the last revision:

 - shrink find_first_bit_impl
 - improve find_last_bit_impl

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/12988/files
  - new: https://git.openjdk.org/jdk/pull/12988/files/c3a75dd1..b3d2aed1

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=12988&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=12988&range=00-01

  Stats: 49 lines in 1 file changed: 16 ins; 12 del; 21 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