[llvm] [IfConversion] Fix duplicate successor error after if-converter. (PR #134485)

[Oliver Stannard via llvm-commits]

ostannard wrote:

I'd prefer to see a test for this before committing it. I'm not very familiar with the if-conversion pass, so it's not clear whether this is the correct fix, or if we should be avoiding doing this optimisation for non-analyzable branches. 

https://github.com/llvm/llvm-project/pull/134485