[llvm] [ValueTracking] Add fast path to avoid second recursive call in `isKnownPositive`; NFC (PR #83638)

[Yingwei Zheng via llvm-commits]

dtcxzyw wrote:

FYI https://dtcxzyw.github.io/llvm-opt-benchmark/coverage/home/dtcxzyw/llvm-project/llvm/lib/Analysis/ValueTracking.cpp.html#L291

https://github.com/llvm/llvm-project/pull/83638