[llvm] [InstCombine] Generate better code for `std::bit_floor` from libstdc++ (PR #144842)

[Yingwei Zheng via llvm-commits]

dtcxzyw wrote:

Currently, SimplifyCFG doesn't convert the branch into select: https://godbolt.org/z/1Kn5aPrz4 So this patch may not have real-world effects.
BTW we don't care about undef-related miscompilation in most cases. Just query `isGuaranteedNotToBeUndef` on `X` is okay if you really care about that. Alive2: https://godbolt.org/z/1Kn5aPrz4


https://github.com/llvm/llvm-project/pull/144842