RFR: 8253420: Refactor HeapRegionManager::find_highest_free [v2]

[Albert Mingkun Yang]

> Using for-loop to make the number of iterations more explicit. Direct backward iteration, `for (uint curr = reserved_length() - 1; curr >= 0; curr--)` doesn't work due to underflow of `uint` type. Therefore, I went for current approach.
> 
> Test: hotspot_gc

Albert Mingkun Yang has updated the pull request incrementally with one additional commit since the last revision:

  review

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Changes:
  - all: https://git.openjdk.java.net/jdk/pull/2193/files
  - new: https://git.openjdk.java.net/jdk/pull/2193/files/c734b63a..67ce558f

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=2193&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=2193&range=00-01

  Stats: 2 lines in 1 file changed: 0 ins; 1 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk/pull/2193.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/2193/head:pull/2193

PR: https://git.openjdk.java.net/jdk/pull/2193