Cast up int instruction code to VALUE

Fix Visual C warnings:
```
iseq.c(3793): warning C4312: 'type cast': conversion from 'int' to 'void *' of greater size
iseq.c(3794): warning C4312: 'type cast': conversion from 'int' to 'void *' of greater size
```

Author: nobu

iseq.c

@@ -3778,7 +3778,7 @@ rb_vm_encoded_insn_data_table_init(void)
     const void * const *table = rb_vm_get_insns_address_table();
 #define INSN_CODE(insn) ((VALUE)table[insn])
 #else
-#define INSN_CODE(insn) (insn)
+#define INSN_CODE(insn) ((VALUE)(insn))
 #endif
     encoded_insn_data = st_init_numtable_with_size(VM_BARE_INSTRUCTION_SIZE);