[code-reflection] RFR: SSA cleanup

[Paul Sandoz]

Refactor the two SSA implementation into distinct classes and reduce public API surface.

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Commit messages:
 - SSA cleanup.

Changes: https://git.openjdk.org/babylon/pull/448/files
  Webrev: https://webrevs.openjdk.org/?repo=babylon&pr=448&range=00
  Stats: 1007 lines in 4 files changed: 352 ins; 644 del; 11 mod
  Patch: https://git.openjdk.org/babylon/pull/448.diff
  Fetch: git fetch https://git.openjdk.org/babylon.git pull/448/head:pull/448

PR: https://git.openjdk.org/babylon/pull/448