[code-reflection] RFR: API for casting Value to Op.Result and Block.Parameter

[Ruby Chen]

This PR adds two methods, `Value::result()` and `Value::parameter()`, which cast a given Value as a Op.Result or Block.Parameter. If the cast fails, the two methods throw an `IllegalStateException`.

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Commit messages:
 - API for casting Value to Op.Result and Block.Parameter

Changes: https://git.openjdk.org/babylon/pull/815/files
  Webrev: https://webrevs.openjdk.org/?repo=babylon&pr=815&range=00
  Stats: 22 lines in 1 file changed: 22 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/babylon/pull/815.diff
  Fetch: git fetch https://git.openjdk.org/babylon.git pull/815/head:pull/815

PR: https://git.openjdk.org/babylon/pull/815