RFR: 8256479: [PPC64] C2 crashes when UseVectorByteReverseInstructionsPPC64 used without SuperwordUseVSX

[Martin Doerr]

C2 crashes on Power9 when using UseVectorByteReverseInstructionsPPC64 without SuperwordUseVSX. bytes_reverse_long_vecNode uses a Vector Register. See bug for more details.

The VectorConversion tests can detect the issue.

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Commit messages:
 - 8256479: [PPC64] C2 crashes when UseVectorByteReverseInstructionsPPC64 used without SuperwordUseVSX

Changes: https://git.openjdk.java.net/jdk/pull/1262/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=1262&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8256479
  Stats: 18 lines in 2 files changed: 16 ins; 0 del; 2 mod
  Patch: https://git.openjdk.java.net/jdk/pull/1262.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/1262/head:pull/1262

PR: https://git.openjdk.java.net/jdk/pull/1262