Fix the order of operands for generator structs

[zhassan-aws]

Description of changes:

While performing codegen for generators, the order of operands was inconsistent with the order of fields for the generator struct. This PR fixes the issue by using the field index to index into the operands array.

Resolved issues:

Resolves #2434

Related RFC:

Optional #ISSUE-NUMBER.

Call-outs:

Testing:

• How is this change tested? Added one test

• Is this a refactor change? No

Checklist

• Each commit message has a non-empty body, explaining why the change was made
• Methods or procedures are documented
• Regression or unit tests are included, or existing tests cover the modified code
• My PR is restricted to a single feature or bugfix

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 and MIT licenses.