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

[Craig Topper via llvm-commits]

topperc 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.

As an experiment I also tried (Zero | One).isAllOnes(). Compile time results are here https://llvm-compile-time-tracker.com/compare.php?from=da8abd84801362080232638abff9d399558cd82a&to=966a5d0e6e70e9ce5e208cc507d2493de5f6dec7&stat=instructions:u

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