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

[Max Graey via llvm-commits]

MaxGraey wrote:

@nikic may I ask you to add this PR ([7bd5aae](https://github.com/llvm/llvm-project/pull/191919/commits/7bd5aaeffb247c23a051ffca097d011ea36dde5c) and [1b92e66](https://github.com/llvm/llvm-project/pull/191919/commits/1b92e666d8effe816fd64c2a21459b33e3409dcf) commits) to llvm-compile-time-tracker runner?

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