[llvm] ConstRange: factor and introduce splitPosNeg (NFC) (PR #126528)

[Nikita Popov via llvm-commits]

https://github.com/nikic commented:

Seems reasonable on the surface, but I don't understand how this is supposed to help with https://github.com/llvm/llvm-project/pull/125899.

https://github.com/llvm/llvm-project/pull/126528