spirv-val invalidated a fuzzed shader because a selection header does not dominates its merge block. According to SPIR-V specification, each header block must strictly dominate its merge block, unless the merge block is unreachable in the CFG.

error: line 172: The selection construct with the selection header 487[%487] does not dominate the merge block 59[%59]
  %59 = OpLabel

Debugging spirv-fuzz, I found that TransformationAddDeadBlock::IsApplicable does not check if existing_block strictly dominate successor_block_id.