Be less conservative about donating atomic instructions

For simplicity, spirv-fuzz is very conservative about donating atomic instructions and does not donate them at all.

However, it might be possible to be less conservative by properly understanding the conditions under which it makes sense to perform an atomic operation, and adapting instructions as needed.

Start by examining spirv-fuzz's handling of atomic instruction donation and the conservatism described in the issue. Determine the conditions that would make donation valid, how instructions would need to be adapted, and what tests would demonstrate that supported atomic operations are donated without invalid transformations.

Written by the indexing model from the issue text.