RFR: 8304042: C2 SuperWord: schedule must remove packs with cyclic dependencies

[Emanuel Peter]

I discovered this bug during the bug fix of [JDK-8298935](https://bugs.openjdk.org/browse/JDK-8298935) [PR](https://git.openjdk.org/jdk/pull/12350).

Currently, the SuperWord algorithm only ensures that all `packs` are `isomorphic` and `independent` (additionally memops are `adjacent`).

This is **not sufficient**. We need to ensure that the `packs` do not introduce `cycles` into the graph. Example:

https://github.com/openjdk/jdk/blob/ad580d18dbbf074c8a3692e2836839505b574326/test/hotspot/jtreg/compiler/loopopts/superword/TestIndependentPacksWithCyclicDependency.java#L217-L231

This is also mentioned in the [SuperWord Paper](https://groups.csail.mit.edu/cag/slp/SLP-PLDI-2000.pdf) (2000, Samuel Larsen and Saman Amarasinghe, Exploiting Superword Level Parallelism with Multimedia Instruction Sets):


3.7 Scheduling
Dependence analysis before packing ensures that statements within a group can be executed
safely in parallel. However, it may be the case that executing two groups produces a dependence
violation. An example of this is shown in Figure 6. Here, dependence edges are drawn between
groups if a statement in one group is dependent on a statement in the other. As long as there
are no cycles in this dependence graph, all groups can be scheduled such that no violations
occur. However, a cycle indicates that the set of chosen groups is invalid and at least one group
will need to be eliminated. Although experimental data has shown this case to be extremely rare,
care must be taken to ensure correctness.


**Solution**

Just before scheduling, I introduced `SuperWord::remove_cycles`. It creates a `PacksetGraph`, based on nodes in the `packs`, and scalar-nodes which are not in a pack. The edges are taken from `DepPreds`. We check if the graph can be scheduled without cycles (via topological sort).

**FYI**

I found a further bug, this time I think it happens during scheduling. See [JDK-8304720](https://bugs.openjdk.org/browse/JDK-8304720). Because of that, I had to disable a test case (`TestIndependentPacksWithCyclicDependency::test5`). I also had to require 64 bit, and either `avx2` or `asimd`. I hope we can lift that again once we fix the other bug. The issue is this: the cyclic dependency example can degenerate to non-cyclic ones, that need to reorder the non-vectorized memory operations.

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

Commit messages:
 - higher requirements on tests, else they trigger the other bug
 - Merge branch 'master' into JDK-8304042
 - fix IR rules
 - remove regression test because of another bug
 - allow reduction self-cycles
 - missing reference
 - only dump in debug
 - implemented the fix
 - 8304042: C2 SuperWord: schedule must remove packs with cyclic dependencies

Changes: https://git.openjdk.org/jdk/pull/13078/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=13078&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8304042
  Stats: 817 lines in 5 files changed: 817 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/13078.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/13078/head:pull/13078

PR: https://git.openjdk.org/jdk/pull/13078