ir3: Document (eq), implement helper invocation optimization using it

What does this MR do and why?

It turns out that this flag is an optimization to kill helper invocations created when PIXLOD is set once they are no longer needed (e.g. after the last texture instruction with implicit derivatives executes), which reduces memory traffic from subsequent loads and reduces divergence. This is analogous to optimizations done on several other architectures, so I expect it to have a decent impact.