RFR: Merge jdk16

[Jesper Wilhelmsson]

Forwardport JDK 16 -> JDK 17

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Commit messages:
 - Merge
 - 8260709: C2: assert(false) failed: unscheduable graph
 - 8260585: AArch64: Wrong code generated for shifting right and accumulating four unsigned short integers

The webrevs contain the adjustments done while merging with regards to each parent branch:
 - master: https://webrevs.openjdk.java.net/?repo=jdk&pr=2441&range=00.0
 - jdk16: https://webrevs.openjdk.java.net/?repo=jdk&pr=2441&range=00.1

Changes: https://git.openjdk.java.net/jdk/pull/2441/files
  Stats: 549 lines in 5 files changed: 527 ins; 16 del; 6 mod
  Patch: https://git.openjdk.java.net/jdk/pull/2441.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/2441/head:pull/2441

PR: https://git.openjdk.java.net/jdk/pull/2441