RFR: 8358013: [PPC64] VSX has poor performance on Power8 [v4]

[Martin Doerr]

> Power8 only has limited VSX instructions for the superword optimization and the Vector API and the performance is bad. Let's only use it on Power9 and newer by default. This change excludes the VSX registers from C2 register allocation for Power8. VSX instruction usage gets limited to a few places like intrinsics.
> 
> Note: Power8 is an old processor and performance optimizations for it are no longer planned.

Martin Doerr has updated the pull request incrementally with one additional commit since the last revision:

  Disable some IR rules for Power8.

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Changes:
  - all: https://git.openjdk.org/jdk/pull/25514/files
  - new: https://git.openjdk.org/jdk/pull/25514/files/599a4f36..2014cb21

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=25514&range=03
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=25514&range=02-03

  Stats: 4 lines in 2 files changed: 3 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/25514.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/25514/head:pull/25514

PR: https://git.openjdk.org/jdk/pull/25514