Ensure that non-recursive conversion is identity transformation wrt all types...

Ensure that non-recursive conversion is identity transformation wrt all types of function calls by only failing on unresolved symbols if they're needed.
Simplify code structure all around. Remove the awkward activity analysis that deemed a function parameter as "modified". Consolidate activity analysis by tracking function parameters and returned symbols separately. Strengthen the type inference a little by using more interpret-like constructs.

PiperOrigin-RevId: 183705547