spirv-fuzz: Transformation to replace an irrelevant pointee with another irrelevant pointee

And, once we add this, _maybe_ re-visit how existing transformations select or create irrelevant pointee values, as they should perhaps do the "simplest" thing and rely on this pass to make things more interesting.

The issue names no files, tests, or entry points. Start by locating existing spirv-fuzz transformations that select or create irrelevant pointee values, then determine the intended transformation for replacing one irrelevant pointee with another. Done means the transformation and the possible follow-up simplification of existing transformations have a defined, agreed scope.

Written by the indexing model from the issue text.