Ensure shortcircuiting function body with runtime errors does not introduce issues

[matteojug]

When throwing runtime errors for reused argument in function definition, we are replacing the function body instructions with the runtime error call. We traverse the body expression to check the body is valid, in case there are errors that the typechecker alone cannot catch; we don't have examples of such instances, we do that to play it safe.

We need to double check that this is not creating dangling references or other unexpected behaviours (or ensure that the leaking side effects are not problematic).

Originally posted by @matteojug in #510 (comment)