RFR: 8376220: C2: Refactor the logic to in MemNode::find_previous_store [v6]

[Quan Anh Mai]

> Hi,
> 
> This patch refactors the logic in `MemNode::find_previous_store` and makes a small improvement to `MemNode::detect_ptr_independence`. An IR test accompanies the improvement.
> 
> Please take a look and share your thoughts, thanks a lot.

Quan Anh Mai has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains seven additional commits since the last revision:

 - Add comments, tests
 - Merge branch 'master' into findpreviousstore
 - Add test store the loaded vector
 - Test description
 - Fix test failures
 - Fix null access
 - Refactor the logic in MemNode::find_previous_store

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/29390/files
  - new: https://git.openjdk.org/jdk/pull/29390/files/f48c006c..1d0f1f9d

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=29390&range=05
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=29390&range=04-05

  Stats: 8480 lines in 207 files changed: 1870 ins; 1506 del; 5104 mod
  Patch: https://git.openjdk.org/jdk/pull/29390.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/29390/head:pull/29390

PR: https://git.openjdk.org/jdk/pull/29390