spirv-fuzz: Improve FuzzerPassObfuscateConstants to ignore types and to use synonyms

FactManager::ConstantUniformFacts::GetUniformDescriptorsForConstant could be improved to ignore type ids; FuzzerPassObfuscateConstants could then add bitcasts as needed, replace signed int with unsigned int (and vice-versa), etc.

FuzzerPassObfuscateConstants::Apply can probably use ForEachInstructionWithInstructionDescriptor.

FuzzerPassObfuscateConstants::Apply need not only consider constants (it could include ids that are synonymous with constants), although perhaps this is fine.

The transformations used (e.g. TransformationReplaceBooleanConstantWithConstantBinary) should instead add a synonym. And then the fuzzer pass should apply TransformationReplaceIdWithSynonym. This is what we do in FuzzerPassInterchangeSignednessOfIntegerOperands.

Start with FactManager::ConstantUniformFacts::GetUniformDescriptorsForConstant and FuzzerPassObfuscateConstants::Apply, then compare the synonym flow in FuzzerPassInterchangeSignednessOfIntegerOperands. Done means the obfuscation pass can ignore type IDs, add needed bitcasts, consider synonymous constants, and use TransformationReplaceIdWithSynonym while the named transformations add synonyms.

Written by the indexing model from the issue text.