[LVI] Handle icmp of ashr. (#68010)

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.

Proof of the original combine: https://alive2.llvm.org/ce/z/SfpsvX

This is a port of the review in https://reviews.llvm.org/D151911 to
GitHub.