RFR: 8259706: C2 compilation fails with assert(vtable_index == Method::invalid_vtable_index) failed: correct sentinel value

[Tobias Hartmann]

When trying to backport [JDK-8259339](https://bugs.openjdk.java.net/browse/JDK-8259339) to JDK 11u, I've hit an assert on Sparc that triggers because the slow virtual call to `Unsafe.allocateUninitializedArray0` could be optimized to a static call since the `Unsafe` class is final.

Thanks,
Tobias

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Commit messages:
 - 8259706: C2 compilation fails with assert(vtable_index == Method::invalid_vtable_index) failed: correct sentinel value

Changes: https://git.openjdk.java.net/jdk/pull/2062/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=2062&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8259706
  Stats: 7 lines in 1 file changed: 5 ins; 0 del; 2 mod
  Patch: https://git.openjdk.java.net/jdk/pull/2062.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/2062/head:pull/2062

PR: https://git.openjdk.java.net/jdk/pull/2062