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

[Afanasyev Ivan via llvm-commits]

ivafanas wrote:

@ostannard @ahatanaka @MatzeB 

Could you please review the PR?  
Seems like `IfConversion.cpp` does not have direct owners, and you are the most involved available developers who changed the file.

I do not have write access.
If patch is ok, please, feel free to merge it :)

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