RFR: Reset top bitmap when region is immediately made into trash

[William Kemper]

Resetting the top bitmap pointer for 100% garbage regions avoids having to clear the bitmap (which is already clear for regions with no live objects).

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Commit messages:
 - Reset top bitmap when region is immediately made into trash

Changes: https://git.openjdk.java.net/shenandoah/pull/104/files
 Webrev: https://webrevs.openjdk.java.net/?repo=shenandoah&pr=104&range=00
  Stats: 5 lines in 2 files changed: 0 ins; 3 del; 2 mod
  Patch: https://git.openjdk.java.net/shenandoah/pull/104.diff
  Fetch: git fetch https://git.openjdk.java.net/shenandoah pull/104/head:pull/104

PR: https://git.openjdk.java.net/shenandoah/pull/104