RFR: 8374789: C2: refactor GraphKit code that create AddP nodes for raw memory to use helper method [v6]

[Guanqiang Han]

> This PR refactors GraphKit code that creates AddP nodes for raw memory to use a dedicated helper method. This avoids passing top() explicitly at call sites and improves readability.
> 
> Please review this change. Thanks!
> 
> **Test:** GHA

Guanqiang Han has updated the pull request incrementally with one additional commit since the last revision:

  Adjust code style

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/29666/files
  - new: https://git.openjdk.org/jdk/pull/29666/files/1f13df85..0d37f22e

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=29666&range=05
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=29666&range=04-05

  Stats: 4 lines in 2 files changed: 0 ins; 0 del; 4 mod
  Patch: https://git.openjdk.org/jdk/pull/29666.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/29666/head:pull/29666

PR: https://git.openjdk.org/jdk/pull/29666