RFR: 8278763: Parallel: Remove grows_up/grows_down in PSVirtualSpace

[Albert Mingkun Yang]

Simple change of inlining statically-known value.

Test: hotspot-gc

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Commit messages:
 - grow-up-down

Changes: https://git.openjdk.java.net/jdk/pull/6831/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=6831&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8278763
  Stats: 9 lines in 2 files changed: 0 ins; 6 del; 3 mod
  Patch: https://git.openjdk.java.net/jdk/pull/6831.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/6831/head:pull/6831

PR: https://git.openjdk.java.net/jdk/pull/6831