Fix emitting of function reference types without GC (#5737)

We previously had logic to emit GC types used in the IR as their corresponding
top types when GC was not enabled (so e.g. nullfuncref would be emitted as
funcref), but the logic was not robust enough and non-null function references
were not properly emitted as funcref.

Refactor the relevant code to be more robust and future-proof, and add a test
demonstrating that the lowering works as intended.

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