RFR: 8253420: Refactor HeapRegionManager::find_highest_free

[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

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Commit messages:
 - for

Changes: https://git.openjdk.java.net/jdk/pull/2193/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=2193&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8253420
  Stats: 12 lines in 1 file changed: 2 ins; 6 del; 4 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