RFR: 8371369: Parallel: Relax precondition of PSOldGen::expand_and_allocate

[Albert Mingkun Yang]

Use more precise precondition for heap/old-gen expansion; the same precondition is also used in its sole caller.

Test: tie1-5

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

Commit messages:
 - pgc-expand-assert

Changes: https://git.openjdk.org/jdk/pull/28162/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=28162&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8371369
  Stats: 2 lines in 1 file changed: 0 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/28162.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/28162/head:pull/28162

PR: https://git.openjdk.org/jdk/pull/28162