spirv-reduce: Remove hints such as branch weights

The reducer could:

- remove branch weights
- change loop, selection and function attributes from interesting things back to None
- remove memory operands such as Volatile and Nontemporal

etc.

At the moment these things get left behind, and can leave one wondering whether they are necessary for triggering a bug.

Start with spirv-reduce and its existing handling of branch weights, loop, selection and function attributes, and memory operands such as Volatile and Nontemporal. Determine how these hints are represented during reduction and add coverage for removing them; done means reduced outputs no longer retain unnecessary hints while preserving the triggering bug.

Written by the indexing model from the issue text.