RFR: 8354650: [PPC64] Try to reduce register definitions

[David Briemann]

Defines the 32 vector registers and removes the 64 vector scalar register definitions.

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Commit messages:
 - 8354650: [PPC64] Try to reduce register definitions

Changes: https://git.openjdk.org/jdk/pull/25828/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=25828&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8354650
  Stats: 608 lines in 7 files changed: 1 ins; 194 del; 413 mod
  Patch: https://git.openjdk.org/jdk/pull/25828.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/25828/head:pull/25828

PR: https://git.openjdk.org/jdk/pull/25828