[code-reflection] RFR: Drop unused local variables from generated op method

[Maurizio Cimadamore]

The generated op method contains some local variables that are never used.
This PR fixes that, further reducing the number of generated locals per synthetic op method.
While this is not a groundbreaking improvement, in some complex tests I've seen a reduction of locals up to 20%.

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Commit messages:
 - Initial push

Changes: https://git.openjdk.org/babylon/pull/413/files
  Webrev: https://webrevs.openjdk.org/?repo=babylon&pr=413&range=00
  Stats: 16 lines in 1 file changed: 8 ins; 0 del; 8 mod
  Patch: https://git.openjdk.org/babylon/pull/413.diff
  Fetch: git fetch https://git.openjdk.org/babylon.git pull/413/head:pull/413

PR: https://git.openjdk.org/babylon/pull/413