reworked AST optimizers analysis of side effects during aggregate operations & calls

src/script_opt/ExprOptInfo.h

@@ -7,11 +7,32 @@
 
 namespace zeek::detail {
 
+// Class for tracking whether a given expression has side effects. Currently,
+// we just need to know whether Yes-it-does or No-it-doesn't, so the structure
+// is very simple.
+
+class ExprSideEffects {
+public:
+    ExprSideEffects(bool _has_side_effects) : has_side_effects(_has_side_effects) {}
+
+    bool HasSideEffects() const { return has_side_effects; }
+
+protected:
+    bool has_side_effects;
+};
+
 class ExprOptInfo {
 public:
     // The AST number of the statement in which this expression
     // appears.
     int stmt_num = -1; // -1 = not assigned yet
+
+    auto& SideEffects() { return side_effects; }
+
+protected:
+    // This optional value missing means "we haven't yet determined the
+    // side effects".
+    std::optional<ExprSideEffects> side_effects;
 };
 
 } // namespace zeek::detail