RFR: 8325252: C2 SuperWord: refactor the packset [v6]

[Emanuel Peter]

> I'm refactoring the packset, separating the details of packset-manupulation from the SuperWord algorithm.
> 
> Most importantly: I split it into two classes: `PairSet` and `PackSet`.
> `combine_pairs_to_longer_packs` converts the first into the second.
> 
> I was able to simplify the combining, and remove the pack-sorting.
> I now walk "pair-chains" directly with `PairSetIterator`. One such pair-chain is equivalent to a pack.
> 
> I moved all the `filter / split` functionality to the `PackSet`, which allows hiding a lot of packset-manipulation from the SuperWord algorithm.
> 
> I ran into some issues when I was extending the pairset in `extend_pairset_with_more_pairs_by_following_use_and_def`:
> Using the PairSetIterator changed the order of extension, and that messed with the packing heuristic, and quite a few examples did not vectorize, because we would pack up the wrong 2 nodes out of a choice of 4 (e.g. we would pack `ac bd` instead of `ab cd`). Hence, I now still have to keep the insertion order for the pairs, and this basically means we are extending with a BFS order. Maybe this issue can be removed, if I improve the packing heuristic with some look-ahead expansion approach (but that is for another day [JDK-8309908](https://bugs.openjdk.org/browse/JDK-8309908)).
> 
> But since I already spent some time on some of the packing heuristic (reordering and cost estimate), I did a light refactoring, and added extra tests for MulAddS2I.
> 
> More details are described in the annotations in the code.

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

  more updates for Christian, batch 2

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/18276/files
  - new: https://git.openjdk.org/jdk/pull/18276/files/72b72288..152c66e2

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=18276&range=05
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=18276&range=04-05

  Stats: 70 lines in 3 files changed: 43 ins; 0 del; 27 mod
  Patch: https://git.openjdk.org/jdk/pull/18276.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/18276/head:pull/18276

PR: https://git.openjdk.org/jdk/pull/18276