[code-reflection] Integrated: Post dominator functionality

[Paul Sandoz]

Add functionality to compute immediate post dominators and the post dominator frontier. Also include facilities to obtain a block's predecessor references and successor targets.

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Commit messages:
 - Post dominator functionality.

Changes: https://git.openjdk.org/babylon/pull/437/files
  Webrev: https://webrevs.openjdk.org/?repo=babylon&pr=437&range=00
  Stats: 303 lines in 3 files changed: 299 ins; 0 del; 4 mod
  Patch: https://git.openjdk.org/babylon/pull/437.diff
  Fetch: git fetch https://git.openjdk.org/babylon.git pull/437/head:pull/437

PR: https://git.openjdk.org/babylon/pull/437