RFR: 8359232: [PPC64] C2: Clean up ppc.ad: add instr sizes, remove comments

[David Briemann]

Add missing sizes for some instructions.
Clean up outdated Power7 comments.

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Commit messages:
 - 8359232: [PPC64] C2: Clean up ppc.ad: add instr sizes, remove comments

Changes: https://git.openjdk.org/jdk/pull/25752/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=25752&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8359232
  Stats: 30 lines in 1 file changed: 10 ins; 20 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/25752.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/25752/head:pull/25752

PR: https://git.openjdk.org/jdk/pull/25752