RFR: 8340010: Fix vectorization tests with compact headers

[Emanuel Peter]

This is a followup to:
https://github.com/openjdk/jdk/pull/20677 / [JDK-8305895](https://bugs.openjdk.org/browse/JDK-8305895) Implement JEP 450: Compact Object Headers (Experimental)

@rkennke fixed the vectorization tests in a very rudamentary way. I now took some time to see what exactly is affected. At the time I reviewed the JEP, I thought only very minor cases were affected, like hand-unrolling etc. But it turns out that there are some more important cases that are affected, like **mixed-type loops**, such as **conversion** between types. Another class of affected loops is **hand-unrolled loops**.

**The problem is this:**
Since the offset from array-base to payload has changed (16 -> 12), some vector loads/stores can now no longer be aligned. This means vectorization under `+AlignVector` is not possible.

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

First: only platforms that require strict-alignment are affected (i.e. `+AlignVector` or `Matcher::misaligned_vectors_ok=false`). I filed [JDK-8344424](https://bugs.openjdk.org/browse/JDK-8344424) for this, so the impact can be discussed. **The affected platforms seem to be exceptions**.

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

Now on to fixing the tests, which we need to do now anyway. Some actually were currently failing.

I once ran over `tier1,tier2,tier3,tier4` plus our internal stress testing with `+-AlignVector` and `+-UseCompactObjectHeaders`, collected all failing tests. I also looked at all tests that were already guarding IR rules on `UseCompactObjectHeaders`.

To almost all tests I added runs with `+-AlignVector` and `+-UseCompactObjectHeaders`. We could leave this also to global runs with these flag combinations. But it is rare that we ever run this, so I thought I want to directly run the "interesting" tests with all combinations. This requires extra test runtime, but I think it is warranted.

In a few cases I also added stronger IR rules, in tests that were already affected - just to make sure we have the behavior we want. Some cases would not vectorize for other cases, and I put comments there for future reference.

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

Commit messages:
 - fix LoopCombinedOpTest.java
 - same case again
 - fixup for Christian
 - manual merge
 - fix sizes for LoopCombinedOpTest.java
 - fix IRExample.java
 - fix TestFloatConversionsVector.java
 - fix TestUnorderedReductionPartialVectorization.java
 - fix TestSplitPacks.java
 - fix 2 more tests
 - ... and 17 more: https://git.openjdk.org/jdk/compare/75420e93...b56c88cb

Changes: https://git.openjdk.org/jdk/pull/22199/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=22199&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8340010
  Stats: 922 lines in 15 files changed: 782 ins; 23 del; 117 mod
  Patch: https://git.openjdk.org/jdk/pull/22199.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/22199/head:pull/22199

PR: https://git.openjdk.org/jdk/pull/22199