RFR: 8287738: [PPC64] jdk/incubator/vector/*VectorTests failing

[Martin Doerr]

`PopCountVI` needs to handle several types after [JDK-8284960](https://bugs.openjdk.java.net/browse/JDK-8284960).
(See https://github.com/openjdk/jdk/commit/6f6486e97743eadfb20b4175e1b4b2b05b59a17a.)

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Commit messages:
 - 8287738: [PPC64] jdk/incubator/vector/*VectorTests failing

Changes: https://git.openjdk.java.net/jdk/pull/8998/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=8998&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8287738
  Stats: 29 lines in 3 files changed: 25 ins; 1 del; 3 mod
  Patch: https://git.openjdk.java.net/jdk/pull/8998.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/8998/head:pull/8998

PR: https://git.openjdk.java.net/jdk/pull/8998