RFR: 8336638: Parallel: Remove redundant mangle in PSScavenge::invoke

[Albert Mingkun Yang]

Trivial removing unnecessary clear at start of young-gc. Also move a precondition to the start, as early as possible.

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Commit messages:
 - pgc-to-clear

Changes: https://git.openjdk.org/jdk/pull/20216/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=20216&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8336638
  Stats: 7 lines in 1 file changed: 3 ins; 4 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/20216.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/20216/head:pull/20216

PR: https://git.openjdk.org/jdk/pull/20216