Add explicit return value to Reducer::SameOp()

Helps tools that don't realize the implications of [[noreturn]] on
the Reporter::InternalError() in this case.

src/script_opt/Reduce.cc

@@ -224,8 +224,8 @@ bool Reducer::SameOp(const Expr* op1, const Expr* op2)
 		return true;
 		}
 
-	else
 	reporter->InternalError("bad singleton tag");
+	return false;
 	}
 
 bool Reducer::SameExpr(const Expr* e1, const Expr* e2)