Minor optimization to search (#1170)

In the final version of the ULM, we want to enforce that semantics are
deterministic. We do this using the search feature and the
`--execute-to-branch` option which treats all states with more than one
successor as final states and does not rewrite them further.

I tested this by running the ethereum test suite with this option and
all the tests passed (with a few trivial changes to the semantics). A
minor change resulted from this effort: if only a single state is active
at a given time, we do not need to do the expensive equality check
performed by `erase`. It is sufficient to simply clear the hash set.

runtime/util/search.cpp

@@ -72,7 +72,11 @@ std::unordered_set<block *, hash_block, k_eq> take_search_steps(
   while (!states.empty() && depth != 0) {
     state = states.front();
     states.pop_front();
-    states_set.erase(state);
+    if (states.empty()) {
+      states_set.clear();
+    } else {
+      states_set.erase(state);
+    }
 
     if (depth > 0) {
       depth--;