RFR: 8352765: G1CollectedHeap::expand_and_allocate() may fail to allocate even after heap expansion succeeds

[Ivan Walulya]

Hi,

Please review this change to ensure that G1 provisions for at least one Eden region after a GC when computing the young length target.

The issue reported in the CR occurs at the end of a GC, after successfully expanding the heap, an allocation fails because `policy()->should_allocate_mutator_region()` returns false. This happens because the computation did not properly account for young regions already allocated as survivor regions, leading to an Eden region target of zero.

With this change, we factor in the young regions that have already been allocated as survivor regions and ensure that at least one region is targeted for Eden.

Testing: Tier 1-3
              Reproducer in the CR.

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Commit messages:
 - init
 - init

Changes: https://git.openjdk.org/jdk/pull/24257/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=24257&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8352765
  Stats: 19 lines in 1 file changed: 6 ins; 0 del; 13 mod
  Patch: https://git.openjdk.org/jdk/pull/24257.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/24257/head:pull/24257

PR: https://git.openjdk.org/jdk/pull/24257