[llvm] [InstCombine] Lower flag check pattern to use a bitmask-shift (PR #169557)

[Yingwei Zheng via llvm-commits]

dtcxzyw wrote:

> Does this transform already exist somewhere in LLVM to some degree? Just checking in godbolt Clang trunk already seems to do a very similar transform on the C snippet in the PR description: https://compiler-explorer.com/z/qTbzc9e33

See SimplifyCFGOpt::simplifyBranchOnICmpChain and simplifySwitchLookup

https://github.com/llvm/llvm-project/pull/169557