RFR: 8295069: [PPC64, possibly aarch64] Performance regression after JDK-8290025

[Martin Doerr]

I have a proposal to mitigate the performance regression (see JBS issue) on PPC64 significantly. We get most of the performance back. Please review. May also be relevant for aarch64 which I haven't checked.
I'm checking explicitly for GCs which are known to be safe. Maybe there is a better way to check this.

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Commit messages:
 - 8295069: [PPC64, possibly aarch64] Performance regression after JDK-8290025

Changes: https://git.openjdk.org/jdk/pull/10632/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=10632&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8295069
  Stats: 10 lines in 2 files changed: 6 ins; 1 del; 3 mod
  Patch: https://git.openjdk.org/jdk/pull/10632.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/10632/head:pull/10632

PR: https://git.openjdk.org/jdk/pull/10632