RFR: 8306440: Rename PSS:_num_optional_regions to _num_initial_optional_regions

[Thomas Schatzl]

Hi all,

   please review this small change that clarifies the use of `PSS::_num_optional_regions` to be the initial amount of optional regions, not the current amount of optional regions.

Testing: gha

Thanks,
  Thomas

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Depends on: https://git.openjdk.org/jdk/pull/13531

Commit messages:
 - Rename PSS:_num_optional_regions to _num_initial_optional_regions to emphasize that it's the initial amount of optional regions

Changes: https://git.openjdk.org/jdk/pull/13532/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=13532&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8306440
  Stats: 10 lines in 3 files changed: 1 ins; 0 del; 9 mod
  Patch: https://git.openjdk.org/jdk/pull/13532.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/13532/head:pull/13532

PR: https://git.openjdk.org/jdk/pull/13532