fix independence bug in create_pack logic in SuperWord::find_adjacent_refs

Please change the generic bug title to something more descriptive once the root cause is known.

The attached Java Fuzzer test results in a wrong execution with C2 compared to a run with the interpreter/C1:

To reproduce:
$ java -Xint Test.java > interpreter.log
$ java -XX:-TieredCompilation -Xcomp -XX:CompileOnly=Test Test.java > c2.log
$ diff interpreter.log c2.log

$ java -Xint Reduced.java > interpreter.log
$ java -XX:-TieredCompilation -Xcomp -XX:CompileOnly=Reduced Reduced.java > c2.log
$ diff interpreter.log c2.log

Output of Test.java diff:
0a1
> CompileCommand: compileonly Test.* bool compileonly = true
10c11
< vMeth_check_sum: 2660033801921391608
---
> vMeth_check_sum: 2660033801921192200
21c22
< vMeth_check_sum: 5320067603842783216
---
> vMeth_check_sum: 5320067603842332488
32c33
< vMeth_check_sum: 7980101405764174824
---
> vMeth_check_sum: 7980101405763472776
43c44
< vMeth_check_sum: -7806608866023985184
---
> vMeth_check_sum: -7806608866024938552
54c55
< vMeth_check_sum: -5146575064102593576
---
> vMeth_check_sum: -5146575064103798264
65c66
< vMeth_check_sum: -2486541262181201968
---
> vMeth_check_sum: -2486541262182657976
76c77
< vMeth_check_sum: 173492539740189640
---
> vMeth_check_sum: 173492539738482312
87c88
< vMeth_check_sum: 2833526341661581248
---
> vMeth_check_sum: 2833526341659622600
98c99
< vMeth_check_sum: 5493560143582972856
---
> vMeth_check_sum: 5493560143580771093
109c110
< vMeth_check_sum: 8153593945504364464
---
> vMeth_check_sum: 8153593945501923741