Avoid double traversal of maps

The typical find/if-not-exists-insert pattern can be achieved with 1
lookup instead of 2 using emplace with a sentinel value. Also maps value
initialize their values when inserted with the [] operator, this is
defined and so there is no need to explicitly insert zeroes for integer
values.

src/V3SplitVar.cpp

@@ -310,7 +310,7 @@ private:
         UASSERT_OBJ(varp->attrSplitVar(), varp, " no split_var metacomment");
         const MapIt it = m_map.find(varp);
         if (it == m_map.end()) return false;  // Not registered
-        const bool ok = m_map[varp].insert(ref).second;
+        const bool ok = it->second.insert(ref).second;
         return ok;
     }