RFR: 8324775: C2 SuperWord: refactor visited sets [v2]

[Emanuel Peter]

> Subtask of https://github.com/openjdk/jdk/pull/16620
> 
> Currently, the visited set is a "global" set that is reused all through SuperWord:
> `_visited`, `_post_visited`, and also `_stk`.
> 
> This makes other refactorings difficult. I am refactoring all related code to make the visited sets local, with use of ResourceMark.
> 
> I am also refactoring the `independent` queries: from using recursive function calls to iterative. This is necessary, unless we want to pass the visited set around into the recursive calls (I think not!)
> 
> At the same time, I wanted the code of `find_dependence` to become easier, and closer to `independent`, hence I re-defined it to `mutually_independent`, and had to slightly adapt also its usages.

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

  comment improvement by Vladimir

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/17594/files
  - new: https://git.openjdk.org/jdk/pull/17594/files/220fc173..59a9205e

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=17594&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=17594&range=00-01

  Stats: 2 lines in 2 files changed: 0 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/17594.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/17594/head:pull/17594

PR: https://git.openjdk.org/jdk/pull/17594