RFR: Merge jdk16

[Jesper Wilhelmsson]

Forwardport JDK 16 -> JDK 17

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Commit messages:
 - Merge
 - 8253505: JFR: onFlush invoked out of order with a sorted event stream
 - 8255120: C2: assert(outer->outcnt() >= phis + 2 && outer->outcnt() <= phis + 2 + stores + 1) failed: only phis

The webrevs contain the adjustments done while merging with regards to each parent branch:
 - master: https://webrevs.openjdk.java.net/?repo=jdk&pr=2087&range=00.0
 - jdk16: https://webrevs.openjdk.java.net/?repo=jdk&pr=2087&range=00.1

Changes: https://git.openjdk.java.net/jdk/pull/2087/files
  Stats: 119 lines in 11 files changed: 43 ins; 40 del; 36 mod
  Patch: https://git.openjdk.java.net/jdk/pull/2087.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/2087/head:pull/2087

PR: https://git.openjdk.java.net/jdk/pull/2087