Add some missing return statements. (#2533)

Author: chriseth

test_data/asm/book/operations_and_links.asm

@@ -18,6 +18,8 @@ machine Main with degree: 16 {
        assert_eq A, 10;
        A <== add4(2, 2, 2, 2);
        assert_eq A, 8;
+
+       return;
     }
 }
 

test_data/std/arith256_memory_large_test.asm

@@ -979,5 +979,7 @@ machine Main with degree: main_degree {
 
         // Left out these test cases, because the format is different...
         // https://github.com/0xPolygonHermez/zkevm-proverjs/blob/a4006af3d7fe4a57a85500c01dc791fb5013cef0/test/sm/sm_arith.js#L1196-L1211
+
+        return;
     }
 }

test_data/std/arith256_small_test.asm

@@ -242,5 +242,7 @@ machine Main with degree: main_degree {
             0xffff, 0xffff, 0xffff, 0xffff, 0xffff, 0xffff, 0xffff, 0xffff, 0xffff, 0xffff, 0xffff, 0xffff, 0xffff, 0xffff, 0xffff, 0xffff);
         assert_eq t_0_0, t_0_1, t_0_2, t_0_3, t_0_4, t_0_5, t_0_6, t_0_7, t_0_8, t_0_9, t_0_10, t_0_11, t_0_12, t_0_13, t_0_14, t_0_15, 1, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0;
         assert_eq t_1_0, t_1_1, t_1_2, t_1_3, t_1_4, t_1_5, t_1_6, t_1_7, t_1_8, t_1_9, t_1_10, t_1_11, t_1_12, t_1_13, t_1_14, t_1_15, 0xfffe, 0xffff, 0xffff, 0xffff, 0xffff, 0xffff, 0xffff, 0xffff, 0xffff, 0xffff, 0xffff, 0xffff, 0xffff, 0xffff, 0xffff, 0xffff;
+
+        return;
     }
 }

test_data/std/arith_large_test.asm

@@ -829,5 +829,6 @@ machine Main with degree: main_degree {
         // Left out these test cases, because the format is different...
         // https://github.com/0xPolygonHermez/zkevm-proverjs/blob/a4006af3d7fe4a57a85500c01dc791fb5013cef0/test/sm/sm_arith.js#L1196-L1211
 
+        return;
     }
 }

test_data/std/arith_small_test.asm

@@ -69,5 +69,6 @@ machine Main with degree: main_degree {
         t_0_0, t_0_1, t_1_0, t_1_1 <== div(0xabcd, 0xef01, 0, 0);
         assert_eq t_0_0, t_0_1, t_1_0, t_1_1, 0, 0, 0xabcd, 0xef01;
 
+        return;
     }
 }