spirv-val invalidated a fuzzed shader because the result id of an OpConstantNull instruction was being used to index into a structure. According to SPIR-V specification, when indexing into a structure, each index must be the result id of an OpConstant instruction.

error: line 79: The <id> passed to OpInBoundsAccessChain to index into a structure must be an OpConstant.
  %399 = OpConstantNull %uint

Debugging spirv-fuzz, I found that TransformationReplaceIdWithSynonym checks only if the index instruction is OpAccessChain, while the same is true for OpInBoundsAccessChain and OpPtrAccessChain.