[code-reflection] RFR: Hat refactor callgraphs and entrypoints

[Gary Frost]

Initial cleanup of callgraphs and entrypoints. Preparation for more comprehensive refactoring for callsite caching and kernel local range specialization.

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

Commit messages:
 - Removing unused method refs from method calls
 - Simplify access to entrypoint names
 - First pass at refactoring entrypoints and callgraphs

Changes: https://git.openjdk.org/babylon/pull/878/files
  Webrev: https://webrevs.openjdk.org/?repo=babylon&pr=878&range=00
  Stats: 171 lines in 19 files changed: 60 ins; 36 del; 75 mod
  Patch: https://git.openjdk.org/babylon/pull/878.diff
  Fetch: git fetch https://git.openjdk.org/babylon.git pull/878/head:pull/878

PR: https://git.openjdk.org/babylon/pull/878