spirv-fuzz: Consider whether it makes sense to support donation of module that use different external instruction sets

At present, the module donation pass in spirv-fuzz assumes that a donor module uses a subset of the external instruction sets that the recipient module uses.

It might be fine to relax this - e.g. to allow a donor that uses OpenCL instructions to be transplanted into a module that does not yet use those instructions, but this requires some thought.

Start by locating the spirv-fuzz module donation pass and its handling of external instruction sets. Compare donor and recipient compatibility, including the OpenCL example, and determine whether differing instruction sets should be supported. Done requires a settled behavior and corresponding implementation and tests; the issue names no files or tests.

Written by the indexing model from the issue text.