spirv-fuzz: Add relaxed decoration to any suitable irrelevant id

We have a fuzzer pass FuzzerPassAddRelaxedDecorations that adds the Relaxed decoration to ids defined in dead blocks.

This could be extended to add the decoration to any id for which the "id is irrelevant" fact holds.

Start by reading the FuzzerPassAddRelaxedDecorations pass and how the “id is irrelevant” fact is represented. Extend the pass beyond ids defined in dead blocks, and verify that suitable irrelevant ids receive the Relaxed decoration without breaking the existing behavior.

Written by the indexing model from the issue text.