RFR: 8289051: C2: Cleanup PhaseCCP::analyze()

[Christian Hagedorn]

As part of [JDK-8288683](https://bugs.openjdk.org/browse/JDK-8288683), which went into JDK 19, I separated out most of the included refactorings into this cleanup RFE to make the fix as small as possible.

This cleanup includes:
- More comments and better descriptions.
- Renamed some variables.
- Split code into multiple methods to improve the readability.

Thanks,
Christian

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Commit messages:
 - 8289051: C2: Cleanup PhaseCCP::analyze()

Changes: https://git.openjdk.org/jdk/pull/9833/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=9833&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8289051
  Stats: 190 lines in 2 files changed: 91 ins; 30 del; 69 mod
  Patch: https://git.openjdk.org/jdk/pull/9833.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/9833/head:pull/9833

PR: https://git.openjdk.org/jdk/pull/9833