Side-effects of memory / register loads

[katrinafyi]

It is unclear whether memory and/or register loads should be considered side-effecting. This affects downstream uses and transformations in significant ways. Most notably, side-effecting operations should be excluded from unused code elimination.

Currently, these are treated as not side-effecting, but this causes inaccuracies in the later Alive2 evaluation. Since the loads may be—but are not always—removed, this can create mismatches in the translation validation where it sees an unmatched load() intrinsic on only one side.

To make this more consistent, we would need to define the semantics and implement them across the toolchain. Additionally, it would be nice to verify that other lifters under test are similar in this way.

This issue is to record some thoughts from #21.