RFR: 8325541: C2 SuperWord: refactor filter / split

[Emanuel Peter]

A few filter / split methods are currently bunched together. We should peal them apart so that moving / changing things becomes easier in the future.


combine_packs -> refactored into:
  combine_packs (now really only combines)
  split_packs_for_max_vector_size
  filter_packs_for_power_of_2_size
  filter_packs_for_mutual_independence

filter_packs -> refactored into:
  filter_packs_for_implemented
  filter_packs_for_profitable


Since we have so many "filter" passes, I created a dedicated `SuperWord::filter_packs` method, that takes a `FilterPredicate` lambda. This also allows me now to systematically debug dump any packs that are rejected .

I also took the liberty to enforce a new invariant between passes: there cannot be any `nullptr` in the packlist.
Before the "filter" passes just set some packs to `nullptr`. I now shift all surviving packs down, and truncate the packlist.

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

Commit messages:
 - small update
 - 8325541

Changes: https://git.openjdk.org/jdk/pull/17785/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=17785&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8325541
  Stats: 279 lines in 2 files changed: 134 ins; 59 del; 86 mod
  Patch: https://git.openjdk.org/jdk/pull/17785.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/17785/head:pull/17785

PR: https://git.openjdk.org/jdk/pull/17785