RFR: 8254793: [JVMCI] improve speculation encoding

[Doug Simon]

This PR changes the encoding of a `jdk.vm.ci.hotspot.HotSpotSpeculationLog.HotSpotSpeculation` from a long to an int.
The `Thread::_pending_failed_speculation` field remains as a `jlong` since it is already exposed to JVMCI Java code
already via VMStructs and this PR does not update its usage in Graal.

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Commit messages:
 - 8254793: encode a HotSpotSpeculation in an int instead of a long

Changes: https://git.openjdk.java.net/jdk/pull/667/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=667&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8254793
  Stats: 80 lines in 11 files changed: 54 ins; 2 del; 24 mod
  Patch: https://git.openjdk.java.net/jdk/pull/667.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/667/head:pull/667

PR: https://git.openjdk.java.net/jdk/pull/667