spirv-fuzz: Add transformation and fuzzer for changing memory scope

This transformation (and fuzzer pass) should strengthen the scope of atomic instructions.
For example: `Invocation` can be `Subgroup` and so on, because It is always acceptable to use a wider scope.

Start by reading the existing spirv-fuzz transformations and fuzzer passes related to atomic instructions and memory scope. Define how widening scopes such as Invocation to Subgroup should be selected, then add the transformation and fuzzer pass so atomic instructions are strengthened while preserving validity; verify the behavior with the relevant existing fuzzer tests.

Written by the indexing model from the issue text.