RFR: 8344253: Test java/util/Spliterator/SpliteratorTraversingAndSplittingTest.java failed

[Viktor Klang]

This change now halves each side of the split regardless, the old code would end up in a situation where it wouldn't decrement since unsigned shift right twice would lead to decrementing estimated size by 0.

It's worth noting the probabilistic nature of skip lists and that their spliterators are not SIZED and their estimatedSize() is therefor not to be relied upon for deterministic correctness.

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Commit messages:
 - Correcting the size estimation for ConcurrentSkipListMap spliterators

Changes: https://git.openjdk.org/jdk/pull/22210/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=22210&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8344253
  Stats: 3 lines in 1 file changed: 0 ins; 0 del; 3 mod
  Patch: https://git.openjdk.org/jdk/pull/22210.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/22210/head:pull/22210

PR: https://git.openjdk.org/jdk/pull/22210