A variant shader was invalidated due an issue in FuzzerPassAddOpPhiSynonyms.
Debugging spirv-fuzz, I found that, in FuzzerPassAddOpPhiSynonyms::GetIdEquivalenceClasses at line 165, pair represents an OpFunction instruction with float return type. But this instruction can not be synonymous with anything. In this case the synonym to be considered is an OpFunctionCall instruction that calls pair.first.

error: line 113: Invalid use of function result id 10[%fx_f1_].
  %262 = OpPhi %float %fx_f1_ %56