[jdk21u-dev] RFR: 8343426: ConcurrentSkipListMap.spliterator() can no longer split the stream

[Satyen Subramaniam]

Backporting JDK-8344253: Test java/util/Spliterator/SpliteratorTraversingAndSplittingTest.java failed. Fixes an intermittent failure with the `java/util/Spliterator/SpliteratorTraversingAndSplittingTest.java failed` test in situations where a decrement wouldn't complete, since unsigned shift right twice would lead to decrementing estimated size by 0. Backporting alongside the related issue: JDK-8344253: https://github.com/openjdk/jdk21u-dev/pull/1895. Ran GHA Sanity Checks, and local Tier 1 and 2. Patch is clean.

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Commit messages:
 - Backport d76b5b888e15b507631068f508e261cab75c841e
 - Backport 2b57f402c46104d4aba784a891ba90604f5e9e4c

Changes: https://git.openjdk.org/jdk21u-dev/pull/1906/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk21u-dev&pr=1906&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8343426
  Stats: 43 lines in 2 files changed: 31 ins; 0 del; 12 mod
  Patch: https://git.openjdk.org/jdk21u-dev/pull/1906.diff
  Fetch: git fetch https://git.openjdk.org/jdk21u-dev.git pull/1906/head:pull/1906

PR: https://git.openjdk.org/jdk21u-dev/pull/1906