Have FunctionBuilder clear the ILBuilder's state in its drop().

lib/frontend/src/ssa.rs

@@ -168,6 +168,13 @@ where
         debug_assert!(self.results.is_empty());
         debug_assert!(self.side_effects.is_empty());
     }
+
+    /// Tests whether an `SSABuilder` is in a cleared state.
+    pub fn is_empty(&self) -> bool {
+        self.variables.is_empty() && self.blocks.is_empty() && self.ebb_headers.is_empty() &&
+            self.calls.is_empty() &&
+            self.results.is_empty() && self.side_effects.is_empty()
+    }
 }
 
 // Small enum used for clarity in some functions.