fix: 'else if' is now lowered into an else > if > else

[Angus-Bethke-Bachmann]

Changed

• 'else if' is now lowered into an else > if > else to ensure functions that return aggregate are not always evaluated when a previous control statement returns true

Testing

• Added a lit test to reproduce the error and confirmed that the error is resolved
• Added a unit test to verify the AST changes