RFR: 8329998: Remove double initialization for parts of small TypeArrays in ZObjArrayAllocator

[Axel Boldt-Christmas]

[JDK-8139457](https://bugs.openjdk.org/browse/JDK-8139457) introduced non-word aligned payloads for TypeArrays. This required adaptation in `ZObjArrayAllocator` segmented array allocator. But the implementation does this adaptation even when the size is small enough and dispatches the allocation to `ObjArrayAllocator`, which already handles clearing this memory.

Instead move the initialization of any unaligned payload start to after the segmented allocation decision.

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Commit messages:
 - 8329998: Remove double initialization for parts of small TypeArrays in ZObjArrayAllocator

Changes: https://git.openjdk.org/jdk/pull/18721/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=18721&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8329998
  Stats: 33 lines in 1 file changed: 16 ins; 13 del; 4 mod
  Patch: https://git.openjdk.org/jdk/pull/18721.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/18721/head:pull/18721

PR: https://git.openjdk.org/jdk/pull/18721