[NewOptimizer] Make stmt_effect_free less aggressive
Previously, the new optimizer used the pre-existing `effect_free` function
to determine whether it is safe to remove an unreferenced statement. However,
this function ignores many builtin's error cases, causing them to be
removed when that is not legal to do (because that would potentially remove
an exception that would otherwise be thrown). Start fixing this, by
introducing a version of the function that is correct for a subset of
intresting functions. We will likely need to expand this when we look
at the benchmarks, but this should be correct for now.
