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

[Max Graey via llvm-commits]

MaxGraey wrote:

The previous implementation (comparison bitwidth with sum of popcounts) allowed conflicts. I decided to keep this behavior, as changing it could potentially something break. `Zero == ~One` would be ideal in terms of simplicity, although it could potentially create an allocation issue for multi-word scenarios. I can try committing it and see how it turns out. Can you add that commit to llvm-compile-time-tracker?

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