spirv-fuzz: Add RMW instructions

Add a transformation (and associated fuzzer pass) to add atomic RMW instructions to the module.

The transformation should cover all of the RMW instructions available with the Shader capability.

We can add an RMW operating on any SSBO or workgroup pointer from a dead block

We can add an RMW operating on any SSBO or workgroup pointer with the "pointee is irrelevant" fact from any block

Start by locating the existing spirv-fuzz transformation and fuzzer-pass implementations that handle instruction insertion and pointer facts. Confirm the design covers every RMW instruction available with the Shader capability and both permitted SSBO/workgroup pointer cases, then verify the associated fuzzer pass exercises them.

Written by the indexing model from the issue text.