RFR: 8254793: [JVMCI] improve speculation encoding [v5]

[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 updated the pull request incrementally with two additional commits since the last revision:

 - require SHA1 algorithm to be present
 - simplified changes such that only the length component of an encode speculation is reduced (to 5 bits)

-------------

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/667/files
  - new: https://git.openjdk.java.net/jdk/pull/667/files/b0d93bdc..c34e25de

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=667&range=04
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=667&range=03-04

  Stats: 60 lines in 5 files changed: 17 ins; 21 del; 22 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