RFR: 8348678: [PPC64] C2: unaligned vector load/store is ok [v4]

[Richard Reingruber]

> This pr changes `Matcher::misaligned_vectors_ok`  to return `true` on PPC64 for better vectorization during `SuperWord`.
> IR checking of the corresponding test `TestCastX2NotProcessedIGVN.java` is also enabled.
> 
> Tested with `TestCastX2NotProcessedIGVN.java`
> 
> The change passed our CI testing:
> Tier 1-4 of hotspot and jdk. All of langtools and jaxp. Renaissance Suite and SAP specific tests.
> Testing was done on the main platforms and also on Linux/PPC64le and AIX.

Richard Reingruber has updated the pull request incrementally with one additional commit since the last revision:

  Copyright header

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Changes:
  - all: https://git.openjdk.org/jdk/pull/23318/files
  - new: https://git.openjdk.org/jdk/pull/23318/files/296adbc6..5071209a

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=23318&range=03
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=23318&range=02-03

  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/23318.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/23318/head:pull/23318

PR: https://git.openjdk.org/jdk/pull/23318