RFR: 8279412: [JVMCI] failed speculations list must outlive any nmethod that refers to it

[Doug Simon]

This PR adds a strong reference from a `HotSpotNmethod` object to the non-empty `HotSpotSpeculationLog` used when compiling the nmethod. This ensures that a `HotSpotSpeculationLog` that [manages](https://github.com/openjdk/jdk/blob/739769c8fc4b496f08a92225a12d07414537b6c0/src/jdk.internal.vm.ci/share/classes/jdk.vm.ci.hotspot/src/jdk/vm/ci/hotspot/HotSpotSpeculationLog.java#L116) a failed speculation list (i.e., the native memory of the failed speculations list is freed after the `HotSpotSpeculationLog` is collected) lives at least as long as the nmethod. Without this guarantee, when the nmethod deopts, it may try add a failed speculation to an already freed list.

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Commit messages:
 - add support for attaching a speculation log to HotSpotNmethod

Changes: https://git.openjdk.java.net/jdk/pull/6949/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=6949&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8279412
  Stats: 41 lines in 2 files changed: 29 ins; 10 del; 2 mod
  Patch: https://git.openjdk.java.net/jdk/pull/6949.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/6949/head:pull/6949

PR: https://git.openjdk.java.net/jdk/pull/6949