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

[Ryan Buchner via llvm-commits]

bababuck wrote:

> Yes. I mean we should move the common logic (ConstantComparesGatherer/switch-to-arithmetic transform) into ValueTracking and reuse it in InstCombine.
 
Understood, thanks for the feedback!

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