RFR: 8317240: Promptly free OopMapEntry after fail to insert the entry to OopMapCache

[Zhengyu Gu]

In OopMapCache::lookup(), after fail to insert newly created OopMapEntry to the cache, the new entry can be free immediately, because it is not published, not accessible by other thread.

The patch also refactored code a bit to reduce duplicate.

Test:
   hotspot_gc on MacOSX (fastdebug and release)

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Commit messages:
 - 8317240: Promptly free OopMapEntry after fail to insert the entry to OopMapCache

Changes: https://git.openjdk.org/jdk/pull/15963/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=15963&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8317240
  Stats: 17 lines in 1 file changed: 6 ins; 5 del; 6 mod
  Patch: https://git.openjdk.org/jdk/pull/15963.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/15963/head:pull/15963

PR: https://git.openjdk.org/jdk/pull/15963