[InstCombine] fold abs(srem X, 2)

This is a missing optimization based on an example in:
https://llvm.org/PR49763

As noted there and the test here, we could add a more
general fold if that is shown useful.

https://alive2.llvm.org/ce/z/xEHdTv
https://alive2.llvm.org/ce/z/97dcY5