RFR: 8271118: C2: StressGCM should have higher priority than frequency-based policy

[Yi Yang]

I think StressGCM should have higher priority than frequency-based policy, otherwise StressGCM has no chance to choose because the frequency-based policy would choose block first if appropriately.

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Commit messages:
 - StressGCM first

Changes: https://git.openjdk.java.net/jdk/pull/4865/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=4865&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8271118
  Stats: 6 lines in 1 file changed: 1 ins; 0 del; 5 mod
  Patch: https://git.openjdk.java.net/jdk/pull/4865.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/4865/head:pull/4865

PR: https://git.openjdk.java.net/jdk/pull/4865