[jdk11u-dev] RFR: 8281811: assert(_base == Tuple) failed: Not a Tuple after JDK-8280799

[Roland Westrelin]

This change doesn't apply cleanly. It requires a GC API method that's not available in 11. There's already adhoc code for shenandoah in PhaseIdealLoop::build_and_optimize() to replace the missing API call. The backported patch uses the same logic. Tested with tier1 and the patch's test case (fails without the fix, passes with it).

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Depends on: https://git.openjdk.java.net/jdk11u-dev/pull/1035

Commit messages:
 - Backport fcce24c5b3cd1dab755fd9b6779af71f4dd78773

Changes: https://git.openjdk.java.net/jdk11u-dev/pull/1049/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk11u-dev&pr=1049&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8281811
  Stats: 96 lines in 3 files changed: 79 ins; 0 del; 17 mod
  Patch: https://git.openjdk.java.net/jdk11u-dev/pull/1049.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk11u-dev pull/1049/head:pull/1049

PR: https://git.openjdk.java.net/jdk11u-dev/pull/1049