[InstCombine] Handle equality comparison when flooring by constant 2

Support `icmp eq` when reducing signed divisions by power of 2 to
arithmetic shift right, as `icmp ugt` may have been canonicalized
into `icmp eq` by the time additions are folded into `ashr`.

Fixes: https://github.com/llvm/llvm-project/issues/73622.

Proof: https://alive2.llvm.org/ce/z/8-eUdb.