Look at stack struct for local variables

[simonlindholm]

This adds missing sp vars that were either only read from (due to being fake and actually part of structs that we fail to detect), or are structs/arrays which never ended up in local_vars because we only generated subfield accesses. Despite adding fake vars I think it's an improvement to have them in the output for the sake of coherence?

(Hard to give project diffs because of how separated declarations are from usages.)