[llvm] [InstCombine] Teach foldSelectOpOp about samesign (PR #122723)

[Alexander Kornienko via llvm-commits]

alexfh wrote:

@nikic @dtcxzyw could you check my reasoning in the previous comment? Am I missing something or is it indeed a miscompile?

https://github.com/llvm/llvm-project/pull/122723