spirv-fuzz: Pure functions

It might be good to know if a particular function is pure (i.e. it doesn't have any side-effects and its output only depends on its input).

This knowledge would allow us to do some interesting transformations. For example, we could permute calls to such functions.

This idea might be related to #3695. In particular, we would need to know if the function writes through any of its pointer parameters.

No files, tests, or entry points are named. Start by reading this issue and related issue #3695, then determine how function purity and writes through pointer parameters should be identified. Done would mean a settled approach for recognizing pure functions so transformations such as permuting their calls can use that information.

Written by the indexing model from the issue text.