RFR: 8371465: Parallel: Revise asserts around heap expansion [v2]

[Albert Mingkun Yang]

> 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

Albert Mingkun Yang has updated the pull request incrementally with one additional commit since the last revision:

  review

-------------

Changes:
  - all: https://git.openjdk.org/jdk/pull/28266/files
  - new: https://git.openjdk.org/jdk/pull/28266/files/f590079b..59d7d1eb

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=28266&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=28266&range=00-01

  Stats: 15 lines in 1 file changed: 12 ins; 1 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/28266.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/28266/head:pull/28266

PR: https://git.openjdk.org/jdk/pull/28266