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

[Max Graey via llvm-commits]

MaxGraey wrote:

The benchmarks have shown some benefits. The question is whether it’s worth making the code a bit more complex. WDYT?

PR is ready for review.

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