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

[Ryan Buchner via llvm-commits]

bababuck wrote:

Moved `ConstantComparesGatherer` from `SimplifyCFG` to `ValueTracking` and re-used that pattern matching from `InstCombine`. I also pulled out the logic for `createBitMapSeq()` and moved that into `ConstantComparesGatherer` (I think this is the wrong place since this logic is for generating a new IR sequence, not Analysis so might not belong in `Analysis/ValueTracking.cpp`, so if someone knows a better spot please let me know!).

Once this is ready I can break up this MR into a few different ones for merging if desired.

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