Fix infinite loops in copy propagation optimizer

[jacob-baines]

I came across a couple of infinite loops in copy_propagation_optimizer.cpp using the attach file.

sslvpnd.zip

The sslvpnd binary should have a sha256sum of 856b68307d53d0f20f5c4a91aefc3146382e4e4210c758655d53135669642557 or you can find the binary embedded in this firmware.

The infinite loop(s) can be reproduced by fetching retdec master from Github, compiling, and executing retdec-decompiler:

albinolobster@ubuntu:~/retdec/build$ retdec-decompiler ~/sslvpnd

After a couple of minutes, retdec drops into an infinite. The issue appears to be similar to 55d46b5f3a4a5ec038ab33331926a195c7f33250, in that copy_progragation_optimizer is walking backwards through statements that loop. I followed a similar solution as introduced in the previous commit, basically exiting the loop if we see the same statement twice. With this change, sslvpnd is successfully processed after approximately 5 minutes.

I looked at the style guide and I'm not 100% my for loop line break is okay or not. Happy to fix if needed.

[s3rvac]

Hi, thanks for the contribution 👍 Let me run TeamCity tests to see if everything passes after the fix.

[s3rvac]

All tests pass and the fix LGTM, so I am going to merge the PR. Thanks again!