RFR: 8348678: [PPC64] C2: unaligned vector load/store is ok [v2]

[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:

  Use -AlignVector by default for POWER8 and later

-------------

Changes:
  - all: https://git.openjdk.org/jdk/pull/23318/files
  - new: https://git.openjdk.org/jdk/pull/23318/files/c75622fa..20dae18d

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=23318&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=23318&range=00-01

  Stats: 3 lines in 1 file changed: 3 ins; 0 del; 0 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