RFR: 8264324: Simplify allocation list management in OopStorage::reduce_deferred_updates

[Kim Barrett]

Please review this cleanup of the allocation list management performed by OopStorage::reduce_deferred_updates.  Part of its job is to take a block whose allocation set has changed and update its place in the allocation list accordingly.  The old code has somewhat confusing logic and does unnecessary work in some cases.  The new code is clearer and avoids that excess work.  (I'm not sure what I was thinking when I wrote the old code.)

Testing:
mach5 tier1
I've also been using this patch for some time under other in-development changes that make heavy use of OopStorage.

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Commit messages:
 - simplify reduce_deferred_updates

Changes: https://git.openjdk.java.net/jdk/pull/3227/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=3227&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8264324
  Stats: 30 lines in 2 files changed: 10 ins; 3 del; 17 mod
  Patch: https://git.openjdk.java.net/jdk/pull/3227.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/3227/head:pull/3227

PR: https://git.openjdk.java.net/jdk/pull/3227