[code-reflection] RFR: ExplicitOnnxOps.If implementation of captured values and initializers

[Adam Sotona]

Deconstruction of `OnnxTransformer` into multiple steps with support for captured values and initializers in nested bodies of `If` op.

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Commit messages:
 - nit change
 - fixed initializers referenced from nested bodies
 - OnnxTransformer and OnnxPartialEvaluator refactoring to support If operator with nested bodies - work in progress
 - OnnxTransformer refactoring
 - if - work in progress

Changes: https://git.openjdk.org/babylon/pull/353/files
  Webrev: https://webrevs.openjdk.org/?repo=babylon&pr=353&range=00
  Stats: 557 lines in 8 files changed: 177 ins; 284 del; 96 mod
  Patch: https://git.openjdk.org/babylon/pull/353.diff
  Fetch: git fetch https://git.openjdk.org/babylon.git pull/353/head:pull/353

PR: https://git.openjdk.org/babylon/pull/353