RFR: 8254793: [JVMCI] improve speculation encoding [v3]

[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.

Doug Simon has refreshed the contents of this pull request, and previous commits have been removed. The incremental
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  8254793: encode a HotSpotSpeculation in an int instead of a long

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Changes:
  - all: https://git.openjdk.java.net/jdk/pull/667/files
  - new: https://git.openjdk.java.net/jdk/pull/667/files/2fb4c99e..2e4e4521

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=667&range=02
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=667&range=01-02

  Stats: 2 lines in 1 file changed: 0 ins; 0 del; 2 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