[llvm] [IRBuilder] Fold binary intrinsics (PR #80743)

[Artem Tyurin via llvm-commits]

agentcooper wrote:

@nikic So the suggestion is to keep the SLPVectorizer code as it is right now and maybe create a separate PR to move SLPVectorizer to TargetFolder instead? Wouldn't you consider it an improvement to extract the existing folding code from SLPVectorizer into IR/ConstantFolder?

https://github.com/llvm/llvm-project/pull/80743