SPIR-V has a Relaxed decoration that can be applied to result ids of certain types to say that they can be computed with relaxed precision (i.e., using fewer bits than normal).

spirv-fuzz needs to preserve the semantics of a SPIR-V module so it cannot add Relaxed to any old id.

However, if an id is in a dead block - a block that has been added by spirv-fuzz but that is guaranteed to be unreachable at runtime - then it is fine to decorate this id with Relaxed.

This issue involved:

Adding a new transformation - TransformationAddRelaxedDecoration - to spirv-fuzz. This should take as an argument the id of an instruction and, to be applicable, should require that the instruction is in a dead block and that the instruction's result type is something to which Relaxed can be applied. When applied it should add the relaxed decoration to the id.
Adding a new fuzzer pass - FuzzerPassAddRelaxedDecorations - that runs as one of the final passes applied by spirv-fuzz. This should - with some probability - add Relaxed to any relevant id.

Some notes for @paulthomson and @Vasniktel for info: I think it might be good - once we have it - to extend this so that Relaxed can be applied to any id that has the arbitrary value fact. That should be an easy change to make later, so I think @antnkarp should go ahead and work on this transformation as described above for now.

spirv-fuzz: transformation to add Relaxed to live safe functions and dead blocks #3502

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