[code-reflection] RFR: Code transformer

[Paul Sandoz]

Rename `OpTransformer` to `CodeTransformer`, and `CopyContext` to `CodeContext`.

Add a simplified operation transformer which is composed into a code transformer, for cases where operations are mostly replaced with one or more other operations (replacing with zero other operations is also possible). In such cases it is not necessary to deal with the code context and value mappings, nor return the block builder, as is required with a code transformer.

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Commit messages:
 - Merge remote-tracking branch 'upstream/code-reflection' into code-transformer
 - Rename OpTransformer to CodeTransformer.

Changes: https://git.openjdk.org/babylon/pull/690/files
  Webrev: https://webrevs.openjdk.org/?repo=babylon&pr=690&range=00
  Stats: 2019 lines in 148 files changed: 461 ins; 220 del; 1338 mod
  Patch: https://git.openjdk.org/babylon/pull/690.diff
  Fetch: git fetch https://git.openjdk.org/babylon.git pull/690/head:pull/690

PR: https://git.openjdk.org/babylon/pull/690