RFR: 8279619: [JVMCI] improve EncodedSpeculationReason

[Doug Simon]

This PR enhances `jdk.vm.ci.meta.EncodedSpeculationReason.encode` such that the `groupName` parameter is included in the encoding. This mitigates the possibility of 2 unrelated speculation objects having the same hash which, in turn, mitigates the possibility of missing a speculation based optimization opportunity.

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Commit messages:
 - EncodedSpeculationReasons with different groupName should produce unique encoded speculations

Changes: https://git.openjdk.org/jdk/pull/12532/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=12532&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8279619
  Stats: 42 lines in 2 files changed: 40 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/12532.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/12532/head:pull/12532

PR: https://git.openjdk.org/jdk/pull/12532