RFR: 8314612: TestUnorderedReduction.java fails with -XX:MaxVectorSize=32 and -XX:+AlignVector [v3]

[Emanuel Peter]

> **Problem**
> 
> There is a case where `PhaseIdealLoop::move_unordered_reduction_out_of_loop` gets a chain of `UnorderedReduction` which have differing vector lengths. But this can lead to wrong results, as some elements now get dropped or "hallucinated".
> 
> Details:
> `-XX:+AlingVector` creates alignment boundaries over which no pack can go. Effectively, it cuts the packs at the alignment boundaries. This can create unexpected series of packs with lengths `[4, 8, 4]`, rather than `[8, 8]`.
> 
> **Solution**
> 
> We need to check that all `UnorderedReduction` have the same `vect_type()`. That ensures all have the same length and element type.
> 
> **Testing**
> 
> I converted the old test into scenario `v016-U`, and added scenario `v032-A` (fails without the patch). Also, I added some vanilla scenarios, just to ensure the test cases can be run with other flag combinations. The IR tests are targeted at scenario `v016-U`.
> 
> Tests: tier1-6, stress-testing. **Running**

Emanuel Peter has updated the pull request incrementally with one additional commit since the last revision:

  add c2 requires

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/15654/files
  - new: https://git.openjdk.org/jdk/pull/15654/files/6c825679..d53ea854

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=15654&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=15654&range=01-02

  Stats: 4 lines in 1 file changed: 4 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/15654.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/15654/head:pull/15654

PR: https://git.openjdk.org/jdk/pull/15654