[code-reflection] RFR: Disallow insertion of a root op in a block

[Mourad Abbay]

A root operation shouldn't be inserted in a block. This changes enforce this rule.

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Commit messages:
 - Disallow insertion of a root op in a block

Changes: https://git.openjdk.org/babylon/pull/425/files
  Webrev: https://webrevs.openjdk.org/?repo=babylon&pr=425&range=00
  Stats: 42 lines in 4 files changed: 41 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/babylon/pull/425.diff
  Fetch: git fetch https://git.openjdk.org/babylon.git pull/425/head:pull/425

PR: https://git.openjdk.org/babylon/pull/425