This handles the case where this combine:
icmp sgt (ashr X, ShAmtC), C --> icmp sgt X, ((C + 1) << ShAmtC) - 1
wasn't performed by instcombine.
This is my first patch to LVI and I'm not really sure this is right.
Proof of the original combine: https://alive2.llvm.org/ce/z/SfpsvX