8371465: Parallel: Revise asserts around heap expansion

[albertnetymk]

Revise heap/old-gen expansion asserts/comments to cover two paths:

1. startup: no gc can occur
2. after-startup: during safepoint, by vm-thread.

Test: tier1-4

---

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• JDK-8371465: Parallel: Revise asserts around heap expansion (Enhancement - P4)

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