RFR: 8325541: C2 SuperWord: refactor filter / split [v2]

[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.

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

  small bugfix

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/17785/files
  - new: https://git.openjdk.org/jdk/pull/17785/files/5bd873b6..d2215242

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=17785&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=17785&range=00-01

  Stats: 2 lines in 1 file changed: 0 ins; 0 del; 2 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