[code-reflection] RFR: Replace opField with opMethod for identified methods

[Mourad Abbay]

In this PR, instead of creating a field that will holds the textual representation of an identified method, we create a method (let's call it opMethod) that will returns the op of that method.

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Commit messages:
 - Replace opField with opMethod for identified methods

Changes: https://git.openjdk.org/babylon/pull/299/files
  Webrev: https://webrevs.openjdk.org/?repo=babylon&pr=299&range=00
  Stats: 65 lines in 5 files changed: 34 ins; 23 del; 8 mod
  Patch: https://git.openjdk.org/babylon/pull/299.diff
  Fetch: git fetch https://git.openjdk.org/babylon.git pull/299/head:pull/299

PR: https://git.openjdk.org/babylon/pull/299