[master] RFR: Fix use of Klass* in GC JFR reporting

[Roman Kennke]

Some JFR related code paths in GC access the Klass* of old objects that has already been overidden by the forwarding pointer. We can easily use the Klass* that we fetched earlier instead.

Testing:
 - [x] tier1
 - [x] tier2
 - [x] jdk/jfr (some of which still fails, see subsequent PRs)

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Commit messages:
 - Fix use of Klass* in GC JFR reporting

Changes: https://git.openjdk.java.net/lilliput/pull/17/files
 Webrev: https://webrevs.openjdk.java.net/?repo=lilliput&pr=17&range=00
  Stats: 16 lines in 4 files changed: 0 ins; 0 del; 16 mod
  Patch: https://git.openjdk.java.net/lilliput/pull/17.diff
  Fetch: git fetch https://git.openjdk.java.net/lilliput pull/17/head:pull/17

PR: https://git.openjdk.java.net/lilliput/pull/17