[llvm] [KnownBits][APInt] Optimize isConstant (NFC) (PR #191919)

[Max Graey via llvm-commits]

MaxGraey wrote:

> As an experiment I also tried (Zero | One).isAllOnes()

I initially thought of doing that too, but for the slow path, this approach would result in memory allocation for the 
`One | Zero` result. That’s why I decided against it

https://github.com/llvm/llvm-project/pull/191919