[code-reflection] RFR: Review FieldLoadOp

[Mourad Abbay]

Make sure FieldLoadOp is consistent with other reflective operations.

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Commit messages:
 - Add resultType param to fieldLoad factory methods

Changes: https://git.openjdk.org/babylon/pull/19/files
 Webrev: https://webrevs.openjdk.org/?repo=babylon&pr=19&range=00
  Stats: 89 lines in 3 files changed: 76 ins; 0 del; 13 mod
  Patch: https://git.openjdk.org/babylon/pull/19.diff
  Fetch: git fetch https://git.openjdk.org/babylon.git pull/19/head:pull/19

PR: https://git.openjdk.org/babylon/pull/19