[llvm] [SimplifyCFG] Extend `simplifySwitchOfPowersOfTwo` to reachable defaults (PR #161807)

[Hans Wennborg via llvm-commits]

zmodem wrote:

Nice!

I worry that `Intrinsic::ctpop` is expensive on targets without a popcount instruction though, such as older x86 and arm without neon support.

Maybe the code should check `TargetTransformInfo::getIntrinsicInstrCost()` first?

https://github.com/llvm/llvm-project/pull/161807