RFR: 8364767: G1: Remove use of CollectedHeap::_soft_ref_policy

[Albert Mingkun Yang]

Use gc-cause checking in `VM_G1CollectFull` to decide soft-ref policy.

Test: tier1-3

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Commit messages:
 - g1-soft-refs

Changes: https://git.openjdk.org/jdk/pull/26648/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=26648&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8364767
  Stats: 42 lines in 6 files changed: 4 ins; 35 del; 3 mod
  Patch: https://git.openjdk.org/jdk/pull/26648.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/26648/head:pull/26648

PR: https://git.openjdk.org/jdk/pull/26648