[llvm] [InstSimplify] Treat float binop with identity as refining (PR #120098)

[Nuno Lopes via llvm-commits]

nunoplopes wrote:

LGTM, thanks!

FWIW if the select is `nnan` then the transformation is correct: https://alive2.llvm.org/ce/z/wsUBYv

https://github.com/llvm/llvm-project/pull/120098