[jdk17u-dev] RFR: 8294902: Undefined Behavior in C2 regalloc with null references

[Goetz Lindenmaier]

I backport this for parity with 17.0.7-oracle.

I had to resolve arena.cpp because there is ASSERT coding in
the context, which already contains the new code.

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Commit messages:
 - Backport 0bbc4181cdbccfc3a542f306ce1902cc2e9f36cb

Changes: https://git.openjdk.org/jdk17u-dev/pull/1106/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk17u-dev&pr=1106&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8294902
  Stats: 65 lines in 7 files changed: 28 ins; 2 del; 35 mod
  Patch: https://git.openjdk.org/jdk17u-dev/pull/1106.diff
  Fetch: git fetch https://git.openjdk.org/jdk17u-dev pull/1106/head:pull/1106

PR: https://git.openjdk.org/jdk17u-dev/pull/1106